Polyolefin composition, its production method, and a battery separator made therefrom

ABSTRACT

The invention relates to a polyolefin composition. The polyolefin composition can be in the form of a multi-layer, microporous polyolefin membrane comprising a first microporous layer containing 7% or less by mass of ultra-high-molecular-weight polyethylene having a weight-average molecular weight of 1×10 6  or more, and having a structure in which a pore size distribution curve obtained by mercury intrusion porosimetry has at least two peaks, and a second microporous layer containing 8% or more by mass of the ultra-high-molecular-weight polyethylene.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a divisional of U.S. application Ser. No. 12/375,877, filed Jan.30, 2009, which is a National Stage of International Application No.PCT/JP2007/065470 filed Aug. 1, 2007, claiming priority based onJapanese Patent Application No. 2006-210307 filed Aug. 1, 2006, thecontents of all of which are incorporated herein by reference in theirentirety.

FIELD OF THE INVENTION

The invention relates to a polyolefin composition. The polyolefincomposition can be in the form of a polyolefin membrane having suitablywell-balanced permeability, mechanical strength, heat shrinkageresistance, meltdown properties, electrolytic solution absorption, andcompression resistance. The invention also relates to a method forproducing such a polyolefin membrane, a battery separator formed by suchpolyolefin membrane, a battery comprising such a separator, and a methodfor using such a battery.

BACKGROUND OF THE INVENTION

Microporous polyolefin membranes are useful as separators for primarybatteries and secondary batteries such as lithium ion secondarybatteries, lithium-polymer secondary batteries, nickel-hydrogensecondary batteries, nickel-cadmium secondary batteries, nickel-zincsecondary batteries, silver-zinc secondary batteries, etc. When themicroporous polyolefin membrane is used as a battery separator,particularly as a lithium ion battery separator, the membrane'sperformance significantly affects the properties, productivity andsafety of the battery. Accordingly, the microporous polyolefin membraneshould have suitably well-balanced permeability, mechanical properties,dimensional stability, shutdown properties, meltdown properties, etc.The term “well-balanced” means that the optimization of one of thesecharacteristics does not result in a significant degradation in another.As is known, it is desirable for the batteries to have a relatively lowshutdown temperature and a relatively high meltdown temperature forimproved battery safety, particularly for batteries exposed to hightemperatures under operating conditions. High separator permeability isdesirable because it generally results in high battery capacity. Aseparator with high mechanical strength is desirable for improvedbattery assembly and fabrication, and for improved durability.

The optimization of material compositions, stretching conditions, heattreatment conditions, etc. has been proposed to improve the propertiesof microporous polyolefin membranes. For example, JP6-240036A disclosesa microporous polyolefin membrane having improved pore diameter and arelatively sharp pore diameter distribution. The membrane is made from apolyethylene resin containing 1% or more by mass ofultra-high-molecular-weight polyethylene having a weight-averagemolecular weight (“Mw”) of 7×10⁵ or more, the polyethylene resin havinga molecular weight distribution (weight-average molecularweight/number-average molecular weight) of 10 to 300, and themicroporous polyolefin membrane having a porosity of 35 to 95%, anaverage penetrating pore size of 0.05 to 0.2 μm, a rupture strength(15-mm width) of 0.2 kg or more, and a pore size distribution (maximumpore size/average penetrating pore size) of 1.5 or less.

WO 2000/20492 discloses a microporous polyolefin membrane havingimproved permeability. The membrane contains fine fibrils made ofpolyethylene having Mw of 5×10⁵ or more or a composition containing suchpolyethylene. The microporous polyolefin membrane has an average poresize of 0.05 to 5 μm, and the percentage of lamellas at angles 0 of 80to 100° relative to a membrane surface being 40% or more in machine andtransverse cross sections.

In general, microporous polyolefin membranes consisting essentially ofpolyethylene (i.e., they contain polyethylene only with no significantpresence of other species) have relatively low meltdown temperatures.Accordingly, proposals have been made to provide microporous polyolefinmembranes made from mixed resins of polyethylene and polypropylene, andmulti-layer, microporous polyolefin membranes having polyethylene layersand polypropylene layers in order to increase meltdown temperature.

WO 2005/113657 discloses a microporous polyolefin membrane havingconventional shutdown properties, meltdown properties, dimensionalstability and high-temperature strength. The membrane is made using apolyolefin composition comprising (a) a polyethylene resin containing 8to 60% by mass of a component having a molecular weight of 10,000 orless, and a Mw/Mn ratio of 11 to 100, wherein Mn is the number-averagemolecular weight of the polyethylene resin, and a viscosity-averagemolecular weight (“Mv”) of 100,000 to 1,000,000, and (b) polypropylene.The membrane has a porosity of 20 to 95%, and a heat shrinkage ratio of10% or less at 100° C. This microporous polyolefin membrane is producedby extruding a melt-blend of the above polyolefin and a membrane-formingsolvent through a die, stretching a gel-like sheet obtained by cooling,removing the membrane-forming solvent, and annealing the sheet.

WO 2004/089627 discloses a microporous polyolefin membrane made ofpolyethylene and polypropylene comprising two or more layers, thepolypropylene content being more than 50% and 95% or less by mass in atleast one surface layer, and the polyethylene content being 50 to 95% bymass in the entire membrane. The membrane has relatively highpermeability and high-temperature strength, as well as a relatively lowshutdown temperature and relatively high short-circuiting temperature.

JP7-216118A discloses a battery separator formed from a porous filmcomprising polyethylene and polypropylene as indispensable componentsand having two microporous layers each with a different polyethylenecontent. The polyethylene content is 0 to 20% by weight in onemicroporous layer, 21 to 60% by weight in the other microporous layer,and 2 to 40% by weight in the overall film. The battery separator hasrelatively high shutdown-starting temperature and mechanical strength.

With respect to the properties of separators, not only permeability,mechanical strength, dimensional stability, shutdown properties andmeltdown properties, but also properties related to battery productivitycharacteristics such as electrolytic solution absorption, and batterycyclability characteristics such as compression resistance have beendisclosed as important. In particular, electrodes for lithium ionbatteries can expand and shrink according to the intrusion and departureof lithium, and an increase in battery capacity can lead to largerexpansion ratios. Because separators are compressed when the electrodesexpand, the separators are needed which suffer little, if any, decreasein electrolytic solution retention by compression. However, when theseparators are provided with larger pore size to achieve improvedelectrolytic solution absorption, the compression resistance of theseparators decrease. Battery separators disclosed in any of JP6-240036A,WO 2000/20492, WO 05/113657, WO 04/089627 and JP7-216118A haveinsufficient electrolytic solution absorption and/or retentioncharacteristics. Thus, microporous polyolefin membranes for batteryseparators are desired which have improved and well-balancedpermeability, mechanical strength, heat shrinkage resistance, meltdownproperties, electrolytic solution absorption, and compressionresistance.

DISCLOSURE OF THE INVENTION

In an embodiment, the invention relates to the discovery of amulti-layer, microporous polyolefin membrane having improvedpermeability, mechanical strength, heat shrinkage resistance, meltdownproperties, and electrolytic solution absorption, and compressionresistance characteristics.

In another embodiment, the invention relates to method for producingsuch a multi-layer, microporous polyolefin membrane.

In another embodiment, the invention relates to a battery separatorformed by such a multi-layer, microporous polyolefin membrane. In otherembodiments, the invention relates to a battery comprising such aseparator, and the use of such a battery. Accordingly, in an embodiment,the multi-layer, microporous polyolefin membrane comprises twomicroporous layers, wherein the first microporous layer comprises afirst microporous layer material, and the second microporous layercomprises a second microporous layer material. The first and secondmicroporous layer materials comprise polyethylene.

The first microporous layer material comprises one of:

-   -   (a) high-density polyethylene (“HDPE”) having a weight-average        molecular weight of about 1×10⁴ to about 5×105;    -   (b) ultra-high-molecular-weight polyethylene (“UHMWPE”) having a        10 weight-average molecular weight of 1×10⁶ or more and HDPE,        the amount of the UHMWPE being 7% or less by mass based on the        combined mass of the UHMWPE and the HDPE;    -   (c) HDPE and polypropylene, the amount of polypropylene being        25% or less by mass based on the combined mass of the HDPE and        the polypropylene; or    -   (d) polypropylene, UHMWPE, and HDPE, the amount of polypropylene        being 25% or less by mass based on the combined mass of the        UHMWPE, the HDPE, and the polypropylene and the amount of UHMWPE        being 7% or less by mass based on the combined mass of the        UHMWPE and the HDPE.

The second microporous layer material comprises one of:

-   -   (a) UHMWPE;    -   (b) UHMWPE and HDPE, the amount of the UHMWPE being at least 8%        by mass based on the combined mass of the UHMWPE and the HDPE;    -   (c) HDPE and polypropylene, the amount of the polypropylene        being 25% or less by mass based on the combined mass of the        polypropylene and the HDPE; or    -   (d) UHMWPE, HDPE, and polypropylene, the amount of polypropylene        being 25% or less by mass based on the mass of the combined mass        of the HDPE, the U′HMWPE, and the polypropylene, and the amount        of the UHNIWPE being at least about 8% by mass based on the        combined mass of the UHMWPE and the HDPE.

Thus, in an embodiment, the invention relates to a two-layer microporouspolyolefin membrane comprising:

-   -   (a) a first microporous layer material constituting a first        microporous layer of the two-layer microporous polyolefin        membrane, the first layer material comprising about 7% or less        by mass of UHMWPE based on the total mass of polyethylene in the        first microporous layer material, wherein the first layer        material is characterized by a structure having a pore size        distribution (e.g., as obtained using mercury intrusion        porosimetry) having at least two peaks; and    -   (b) a second microporous layer material constituting a second        microporous layer of the two-layer microporous polyolefin        membrane, the second microporous layer material comprising at        least about 8% by mass of UHMWPE based on the total mass of the        polyethylene in the second microporous layer material.

In this embodiment, the first layer (which can also be called a firstmicroporous layer) constitutes an outer surface of the membrane and thesecond layer (which can also be called a second microporous layer)constitutes a second outer surface of the membrane. For example, thefirst layer can be the top surface of the membrane and the second layercan be the bottom surface of the membrane (with the membrane orientedhorizontally). Optionally, the first layer is in contact with the secondlayer. The term “in contact with” as used in this and the otherembodiments means the first and second layer are in planar (i.e., notedge-wise) contact. In other words, the planar surface of the firstlayer, for example, has an interface with the planar surface of thesecond layer, with the interface located in the interior of themembrane.

In an embodiment, the multi-layer, microporous polyolefin membranecomprises three or more layers, wherein the first and third microporouslayers constitute outer (or “skin”) layers of the membrane and comprisethe first microporous layer material. The second microporous layerconstitutes a second microporous layer located between the first andthird microporous layers. Optionally, the second microporous layer is incontact with at least one of the first and third microporous layers. Thesecond microporous layer comprises the second microporous layermaterial. The first and second microporous layer materials can be thesame as those described above for the two-layer membrane.

Thus, in an embodiment, the multi-layer, microporous polyolefin membraneof the present invention comprises at least three layers, wherein (a) afirst and a third microporous layer constitute at least both surfacelayers of the multi-layer, microporous polyolefin membrane, the firstand the third microporous layers comprising a first polyethylene,wherein the

-   -   (a) olyethylene comprises 7% or less by mass oft11-1MWFE based        on the mass of the polyethylene in the first microporous layer        material, wherein the first layer material is characterized by a        pore size distribution curve (e.g., as obtained using mercury        intrusion porosimetry) having at least two peaks; and    -   (b) a second microporous layer constituting at least one second        layer located between both the first and third layers, wherein        the second microporous layer comprises a second microporous        layer material comprising 8% or more by mass of UBMWPE based on        the total mass of the polyethylene in the second microporous        layers material.

Accordingly, the first and third layers of the multi-layer, microporouspolyolefin membrane are microporous layers that comprise, or consist of,or consist essentially of the first microporous layer material. Thesecond layer of the multi-layer, microporous polyolefin membrane is amicroporous layer located between the first and third layers. In otherwords, the second layer constitutes an “intermediate” or “middle” or“interior” layer of the multi-layer, microporous polyolefin membrane.The second layer comprises, or consists of, or consists essentially ofthe second microporous layer material. In an embodiment, themulti-layer, microporous polyolefin membrane further comprises aplurality of intermediate layers, with at least one of the intermediatelayers being the second layer, the other intermediate layers comprisingthe first layer material and/or second layer material. Optionally, thefirst microporous layer and third microporous layer (i.e., the layerswhich comprise the first layer material) have an average pore size inthe range of about 0.02 μm to about 0.05 μm, and the second microporouslayer has an average pore size of in the range of about 0.005 to about0.1 μm. The term “pore size” is analogous to the “pore diameter” ofapproximately spherical pores. It should be understood that the pores ofthe multi-layer, microporous polyolefin membrane are not necessarilyspherical, even though in embodiments of the invention the pores areapproximated as spheres for the purpose of measuring, e.g., a pore sizedistribution.

In an embodiment, the first microporous layer material (andconsequently, at least the first layer of the multi-layer, microporouspolyolefin membrane) comprises dense domains having a main peak in thepore size distribution curve in a range of about 0.01 μm to about 0.08μm and coarse domains having at least one sub-peak in a range of about0.08 μm to about 1.5 μm. In another embodiment, the first microporouslayer material comprises dense domains having a main peak in the poresize distribution curve in a range of about greater than 0.08 μm andless than 1.5 μm, in the pore size distribution curve. The ratio of thepore volume of the dense domains (calculated from the main peak) to thepore volume of the coarse domains (calculated from the sub-peak) is notcritical, and can range, e.g., from about 0.5 to about 49. In anembodiment where the multi-layer microporous polyolefin membrane is athree-layer membrane, the thickness ratio expressed as the fractionfirst microporous layer/second microporous layer/third microporous layercan be, for example, about 1/(0.015 to 0.95)/1, where the first andthird layers have a thickness normalized to 1.

The multi-layer, microporous polyolefin membranes have suitablywell-balanced permeability, mechanical strength, heat shrinkageresistance, meltdown properties, electrolytic solution absorption, andcompression resistance characteristics.

In an embodiment, the multi-layer, microporous polyolefin membrane hassurface roughness of at least about 3×10² nm. With surface roughnesswithin this range, the microporous polyolefin membrane has a relativelylarge contact area with an electrolytic solution when used as a batteryseparator, which can lead to relatively high electrolytic solutionabsorption.

In another embodiment, the invention relates to a first method forproducing a multi-layer, microporous polyolefin membrane. The processinvolves producing a first polyolefin solution and a second polyolefinsolution and then extruding the first and second polyolefin solutionsthrough at least one die. The first polyolefin solution comprises afirst membrane-forming solvent and a first polyolefin composition. Thesecond polyolefin composition comprises a second membrane-formingsolvent and a second polyolefin composition. The first and secondpolyolefin compositions contain polyethylene and are generally producedfrom one or more resins containing e.g., polyethylene, and optionallypolypropylene and other species. Accordingly, in an embodiment the firstpolyolefin composition is produced by combining HDPE and UHMWPE resins,wherein the first polyolefin composition contains 7% or less by mass ofUHMWPE based on the mass of the first polyethylene composition. Thesecond polyethylene composition is produced by combining ‘UHMWPE resinand optionally HDPE resin. The second polyolefin composition comprisesat least 8% by mass of UHMWPE based on the mass of the secondpolyethylene composition. The UHMWPE resin and the HDPE resin used inthe first polyolefin composition need not be the same as the UHMWPEresin and the HDPE resin used in the second polyolefin composition.

In an embodiment, the invention relates to a first method for producingthe multi-layer, microporous polyolefin membrane comprises the steps of(1) combining (e.g., by melt-blending) a first polyolefin compositionand a membrane-forming solvent to prepare a first polyolefin solution,(2) combining a second polyolefin composition and a secondmembrane-forming solvent to prepare a second polyolefin solution, (3)extruding (preferably simultaneously) the first and second polyolefinsolutions through at least one die to form an extrudate, (4) cooling theextrudate to form a multi-layer, gel-like sheet, (5) removing themembrane-forming solvent from the multi-layer, gel-like sheet to form asolvent-removed gel-like sheet, and (6) drying the solvent-removedgel-like sheet in order to form the multi-layer, microporous polyolefinmembrane. In step (3), wherein the extruding may comprise followingsteps of (a) at least a portion of the first polyolefin solution isextruded through a first die, (b) at least a portion of the secondpolyolefin solution is co-extruded through a second die, and (c) atleast a portion of the first polyolefin solution is co-extruded througha third die, wherein the extrudate is a multi-layer extrudate whichcomprises a first layer and a third layer comprising the extruded firstpolyolefin solution, and a second layer comprising the extruded secondpolyolefin solution, with the second layer being located between thefirst and third layers. An optional stretching step (7), and an optionalhot solvent treatment step (8), etc. can be conducted between steps (4)and (5), if desired. After step (6), an optional step (9) of stretchinga multi-layer, microporous membrane, an optional heat treatment step(10), an optional cross-linking step with ionizing radiations (11), andan optional hydrophilic treatment step (12), etc., can be conducted ifdesired. The order of the optional steps is not critical.

In another embodiment, the invention relates to a second method forproducing the multi-layer microporous polyolefin membrane. The secondmethod for producing the multi-layer, microporous polyolefin membranecomprises the steps of (1) combining (e.g., by melt-blending) a firstpolyolefin composition and a first membrane-forming solvent to prepare afirst polyolefin solution, (2) combining a second polyolefin compositionand a second membrane-forming solvent to prepare a second polyolefinsolution, (3) extruding the first polyolefin solution through a firstdie and the second solution through a second die and then laminating theextruded first and second polyolefin solutions to form a multi-layerextrudate, (4) cooling the multi-layer extrudate to form a multi-layer,gel-like sheet, (5) removing the membrane-forming solvent from themulti-layer, gel-like sheet to form a solvent-removed gel-like sheet,and (6) drying the solvent-removed gel-like sheet in order to form themulti-layer, microporous membrane. In step (3), wherein the extrudingmay comprise following steps of (a) at least a portion of the firstpolyolefin solution is extruded through the first die to make a firstextrudate, (b) at least a portion of the second polyolefin solution isextruded through the second die to make a second extrudate, and (c) atleast a portion of either the first or second polyolefin solution isextruded through a third die to make a third extrudate, and thenlaminating the first, second, and third extrudates to make a multi-layerextrudate which comprises a first layer and a third layer comprising theextruded first polyolefin solution, and a second layer comprising theextruded second polyolefin, with the second layer being located betweenthe first and third layers. An optional stretching step (7), and anoptional hot solvent treatment step (8), etc., can be conducted betweensteps (4) and (5), if desired. After step (6), an optional step (9) ofstretching a multi-layer, microporous membrane, an optional heattreatment step (10), an optional cross-linking step with ionizingradiations (11), and an optional hydrophilic treatment step (12), etc.,can be conducted.

In another embodiment, the invention relates to a third method forproducing the multi-layer microporous polyolefin membrane. The thirdmethod for producing the multi-layer, microporous polyolefin membranecomprises the steps of (1) combining (e.g., by melt-blending) a firstpolyolefin composition and a membrane-forming solvent to prepare a firstpolyolefin solution, (2) combining a second polyolefin composition and asecond membrane-forming solvent to prepare a second polyolefin solution,(3) extruding the first polyolefin solution through at least one firstdie to form at least one first extrudate, (4) extruding the secondpolyolefin solution through at least one second die to form at least onesecond extrudate, (5) cooling first and second extrudates to form atleast one first gel-like sheet and at least one second gel-like sheet,(6) laminating the first and second gel-like sheet to form amulti-layer, gel-like sheet, (7) removing the membrane-forming solventfrom the resultant multi-layer, gel-like sheet to form a solvent-removedgel-like sheet, and (8) drying the solvent-removed gel-like sheet inorder to form the multi-layer, microporous membrane. The steps (3)-(6)may comprise following steps of (3) extruding at least a portion of thefirst polyolefin solution through a first and third die to make a firstand third extrudate, (4) extruding at least a portion of the secondpolyolefin solution through a second die to make a second extrudate, (5)cooling first, second, and third extrudates to form first, second, andthird gel-like sheets, and (6) laminating the first, second, and thirdgel-like sheets to form a multi-layer, gel-like sheet. An optionalstretching step (9), and an optional hot solvent treatment step (10),etc., can be conducted between steps (5) and (6) or between steps (6)and (7), if desired. After step (8), an optional step (11) of stretchinga multi-layer, microporous membrane, an optional heat treatment step(12), an optional cross-linking step with ionizing radiations (13), andan optional hydrophilic treatment step (14), etc, can be conducted.

The main difference between the third production method and the secondproduction method is in the order of the steps for laminating andcooling. In the second production method, laminating the first andsecond polyolefin solutions is conducted before the cooling step. In thethird production method, the first and second polyolefin solutions arecooled before the laminating step.

In another embodiment, the invention relates to a fourth method forproducing the multi-layer microporous polyolefin membrane. The fourthmethod for producing the multi-layer, microporous polyolefin membranecomprises the steps of (1) combining (e.g., by melt-blending) a firstpolyolefin composition and a membrane-forming solvent to prepare a firstpolyolefin solution, (2) combining a second polyolefin composition and asecond membrane-forming solvent to prepare a second polyolefin solution,(3) extruding the first polyolefin solution through at least one firstdie to form at least one first extrudate, (4) extruding the secondpolyolefin solution through at least one second die to form at least onesecond extrudate, (5) cooling first and second extrudates to form atleast one first gel-like sheet and at least one second gel-like sheet,(6) removing the first and second membrane-forming solvents from thefirst and second gel-like sheets to form solvent-removed first andsecond gel-like sheets, (7) drying the solvent-removed first and secondgel-like sheets to form at least one first polyolefin membrane and atleast one second polyolefin membrane, and (8) laminating the first andsecond microporous polyolefin membranes in order to form themulti-layer, microporous polyolefin membrane.

The steps (3)-(8) may comprise following steps of (3) extruding at leasta portion of the first polyolefin solution through a first and third dieto make a first and third extrudate, (4) extruding at least a portion ofthe second polyolefin solution through a second die to make a secondextrudate, (5) cooling first, second, and third extrudates to formfirst, second, and third gel-like sheets, (6) removing the first andsecond membrane-forming solvents from the first, second, and thirdgel-like sheets, (7) drying the solvent-removed first, second, and thirdgel-like sheets to form first, second, and third microporous polyolefinmembranes, and (8) laminating the first, second, and third microporouspolyolefin membranes in order to form the multi-layer, microporouspolyolefin membrane. A stretching step (9), a hot solvent treatment step(10), etc., can be conducted between steps (5) and (6), if desired. Astretching step (11), a heat treatment step (12), etc., can be conductedbetween steps (7) and (8), if desired. After step (8), a step (13) ofstretching a multi-layer, microporous membrane, a heat treatment step(14), a cross-linking step with ionizing radiations (15), a hydrophilictreatment step (16), etc., can be conducted if desired.

The following are preferred embodiments of the present invention:

(1) The method for making a multi-layer, microporous polyolefinmembrane, wherein the first polyolefin solution does not containultra-high molecular weight polyethylene.(2) The method for making a multi-layer, microporous polvolefinmembrane, wherein the second polyolefin composition comprises highdensity polyethylene and ultra-high molecular weight polyethylene.(3) The method for making a multi-layer, microporous polyolefinmembrane, wherein the first and second polypropylenes are independentlyselected from polypropylene having a molecular weight ranging from about1×10⁴ to about 4×10⁶.(4) The method for making a multi-layer, microporous polvolefinmembrane, wherein the first membrane-forming solvent comprises one ormore of (i) aliphatic, alicyclic or aromatic hydrocarbons; (ii) mineraloil distillates having boiling points comparable to the aliphatic,alicyclic or aromatic hydrocarbons; (iii) stearyl alcohol, (iv) cerylalcohol, and (v) paraffin waxes.(5) The method for making a multi-layer, microporous polyolefinmembrane, wherein the second membrane-forming solvent comprises one ormore of (i) aliphatic, alicyclic or aromatic hydrocarbons; (ii) mineraloil distillates having boiling points comparable to the aliphatic,alicyclic or aromatic hydrocarbons; (iii) stearvl alcohol, (iv) cerylalcohol, and (v) paraffin waxes.

The battery separator according to the present invention, which is madeby the multi-layer, microporous polyolefin membrane described above.

The battery according to the present invention, which is made by theseparator described above. The battery may contain an anode, a cathode,and at least one separator comprising the multi-layer, microporouspolyolefin membrane located between the anode and the cathode. Thebattery may be a secondary battery. The battery may be a lithium-ionsecondary battery.

In yet another embodiment, the invention relates to a method for usingthe battery as a source or sink of electric charge.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a graph showing a typical pore size distribution curve.

FIG. 2 is a schematic view showing a method for measuring a meltdowntemperature.

DETAILED DESCRIPTION OF THE INVENTION [1] Composition of the Multi-LayerMicroporous 10 Polyolefin Membrane

In an embodiment, the multi-layer, microporous polyolefin membranecomprises two layers. The first layer (e.g., the top or upper layer ofthe membrane) comprises a first microporous layer material, and thesecond layer (e.g., the bottom or lower layer of the membrane) comprisesa second microporous layer material. For example, the membrane can havea planar top layer when viewed from above on an axis approximatelyperpendicular to the transverse and machine (longitudinal) directions ofthe membrane, with the bottom planar layer hidden from view by the toplayer. In another embodiment, the multi-layer, microporous polyolefinmembrane comprises three or more layers, wherein the outer layers (alsocalled the “surface” or “skin” layers) comprise the first microporouslayer material and at least one intermediate layer comprises the secondmicroporous layer material. In a related embodiment, where themulti-layer, microporous polyolefin membrane comprises two layers, thefirst layer consists essentially of (or consists of) the firstmicroporous layer material and the second layer consists essentially of(or consists of) the second microporous layer material. In a relatedembodiment where the multi-layer, microporous polyolefin membranecomprises three or more layers, the outer layers consist essentially of(or consist of) the first microporous layer material and at least oneintermediate layer consists essentially of (or consists of) the secondmicroporous layer material.

The first and second microporous layer materials contain polyethylene.The first and second microporous layer materials will now be describedin more detail.

A. The First Microporous Layer Material

In an embodiment, the first microporous layer material comprises one of:

(i) a first polyethylene having an Mw that is less than 1×106;

(ii) the first polyethylene and a second polyethylene having a Mw of atleast about 1×10⁶, wherein the second polyethylene is present in anamount that does not exceed about 7% or by mass based on the combinedmass of the first and second polyethylene;

(iii) the first polyethylene and a first polypropylene, wherein theamount of the polypropylene ranges does not exceed about 25% by massbased on the combined mass of the first polyethylene and the firstpolypropylene; or

(iv) the first polyethylene, the second polyethylene, and the firstpolypropylene, wherein the first polypropylene is present in an amountthat does not exceed about 25% by mass based on the combined mass of thefirst polyethylene, the second polyethylene, and the firstpolypropylene, and wherein the second polyethylene is present in anamount that does not exceed about 7% by mass based on the combined massof the first and second polyethylene.

In an embodiment, the first microporous layer material (andconsequently, the first layer of the two-layer, microporous polyolefinmembrane and the first and third layers of a three-layer microporouspolyolefin membrane) is characterized by a pore size distributionexhibiting relatively dense domains having a main peak in a range of0.01 μm to 0.08 μm and relatively coarse domains exhibiting at least onesub-peak in a range of more than 0.08 μm to 1.5 μm or less in the poresize distribution curve. The ratio of the pore volume of the densedomains (calculated from the main peak) to the pore volume of the coarsedomains (calculated from the sub-peak) is not critical, and can range,e.g., from about 0.5 to about 49. Generally, the dense domains andcoarse domains are irregularly entangled to form a hybrid structure inany cross sections of the first microporous layer as viewed in machineand transverse directions.

The first microporous layer material can contain the secondpolyethylene. Provided it does not exceed 7% by mass based on the massof the first microporous layer material, the amount of the secondpolyethylene in the first microporous layer material in not critical.When the amount of the second polyethylene (when present) in the firstmicroporous layer material is more than 7% by mass, it can be moredifficult to produce a first microporous layer material exhibiting ahybrid structure.

The Mw of the polyolefin in the first microporous layer material is notcritical, and can be e.g., about 1×10⁶ or less. In an embodiment, the Mwof the polyolefin in the first microporous layer material ranges fromabout 1×15 10⁵ to about 1×10⁶, or from about 2×10⁵ to about 1×10⁶. Whenthe Mw of the polyolefin in the first layer material is more than 1×10⁶,it can be more difficult to produce a first microporous layer materialexhibiting a hybrid structure. When the Mw of the polyolefin in thefirst layer material is less than 1×10⁵, it is more difficult to producea multi-layer, microporous polyolefin membrane that does not break ortear during stretching.

When the first layer material contains polypropylene, the amount ofpolypropylene can be, e.g., about 25% or less by mass, or in a rage ofabout 2% to about 15% by mass, or in a rage of about 3% to about 10% bymass, based on 100% by mass of the first microporous layer material.

B. The Second Microporous Layer Material

In an embodiment, the second microporous layer material comprises oneof:

-   -   (i) a fourth polyethylene having an Mw of at least about 1×106;    -   (ii) a third polyethylene having an Mw that is less than 1×10⁶        and the fourth polyethylene, wherein the fourth polyethylene is        present in an amount of at least about 8% by mass based on the        combined mass of the third and fourth polyethylene;    -   (iii) the fourth polyethylene and a second polypropylene wherein        the second polypropylene is present in an amount that does not        exceed about 25% by mass based on the combined mass of the        fourth polyethylene and the second polypropylene; or    -   (iv) the third polyethylene, the fourth polyethylene, and the        second polypropylene, wherein second polypropylene is present in        an amount that does not exceed about 25% by mass based on the        combined mass of the third polyethylene, the fourth        polyethylene, and the second polypropylene, and the fourth        polyethylene is present in an amount of at least about 8% by        mass based on the combined mass of the third and fourth        polyethylene.

In an embodiment, the amount of the second polyethylene in the secondmicroporous layer material is at least about 8% by mass, based on 100%by mass of the total amount of polyethylene in the second microporouslayer material. When this amount is less than 8% by mass, it can be moredifficult to produce a relatively strong multi-layer, microporouspolyolefin membrane. This amount is not critical (provided it is atleast about 8%), and can be, e.g., at least 20% by mass, or at leastabout 25% by mass.

In an embodiment where the second microporous layer material comprisespolyethylene and polypropylene, the amount of polypropylene can be,e.g., about 25% or less by mass based on 100% by mass of the secondmicroporous layer material. When this amount is more than 25% by mass,it is more difficult to produce a relatively strong multi-layer,microporous polyolefin membrane. Optionally, the amount of polypropylenein the second microporous layer material ranges, e.g., from about 2% toabout 15% by mass, or from about 3% to about 10% by mass.

C. The First Polyethylene

In an embodiment, the first polyethylene is a polyethylene having an Mwranging from about 1×10⁴ to about 5×10⁵. For example, the firstpolyethylene can be one or more of a high-density polyethylene, amedium-density polyethylene, a branched low-density polyethylene, or alinear low-density polyethylene. Although it is not critical, the Mw ofhigh-density polyethylene can range, for example, from about 1×10⁵ toabout 5×10⁵, or from about 2×10⁵ to about 4×10⁵. In an embodiment, thefirst polyethylene is at least one of (i) an ethylene homopolymer or(ii) a copolymer of ethylene and a third a-olefin such as propylene,butene-1, hexene-1, etc, typically in a relatively small amount comparedto the amount of ethylene. Such a copolymer can be produced using asingle-site catalyst.

D. The Second Polyethylene

The second polyethylene is a polyethylene having an Mw of at least about1×10⁶. In an embodiment, the second polyethylene is at least one of (i)an ethylene homopolymer or (ii) a copolymer of ethylene and a fourtha-olefin which is typically present in a relatively small amountcompared to the amount of ethylene. The fourth a-olefin can be, forexample, one or more of propylene, butene-1, pentene-1,hexene-1,4-methylpentene-1, octene-1, vinyl acetate, methylmethacrylate, or styrene. Although it is not critical, the Mw of thesecond polyethylene can range from about 1×10⁶ to about 15×10⁶, or fromabout 1×10⁶ to about 5×10⁶, or from about 1×10⁶ to about 3×106.

E. The Third Polyethylene

In an embodiment, the third polyethylene is a polyethylene having an Mwranging from about 1×10⁴ to about 5×10⁵. For example, the thirdpolyethylene can be one or more of a high-density polyethylene, amedium-density polyethylene, a branched low-density polyethylene, or alinear low-density polyethylene. Although it is not critical, the Mw ofhigh-density polyethylene can range, for example, from about 1×10⁵ toabout 5×10⁵, or from about 2×10⁵ to about 4×10⁵. In an embodiment, thethird polyethylene is at least one of (i) an ethylene homopolymer or(ii) a copolymer of ethylene and a third a-olefin such as propylene,butene-1, hexene-1, etc, typically in a relatively small amount comparedto the amount of ethylene. Such a copolymer can be produced using asingle-site catalyst. The third polyethylene can be the same as thefirst polyethylene, but this is not required.

F. The Fourth Polyethylene

The fourth polyethylene is a polyethylene having an Mw of at least about1×10⁶. In an embodiment, the fourth polyethylene is at least one of (i)an ethylene homopolymer or (ii) a copolymer of ethylene and a fourtha-olefin which is typically present in a relatively small amountcompared to the amount of ethylene. The fourth a-olefin can be, forexample, one or more of propylene, butene-1, pentene-1,hexene-1,4-methylpentene-1, octene-1, vinyl acetate, methylmethacrylate, or styrene. Although it is not critical, the Mw of thefourth polyethylene can range from about 1×10⁶ to about 15×10⁶, or fromabout 1×10⁶ to about 5×10⁶, or from about 1×10⁶ to about 3×10⁶ Thefourth polyethylene can be the same as the second polyethylene, but thisis not required

G. The First Polypropylene

The first microporous layer materials can optionally comprise a firstpolypropylene. The polypropylene can be, for example, one or more of (i)a propylene homopolymer or (ii) a copolymer of propylene and a fiftholefin. The copolymer can be a random or block copolymer. The fiftholefin can be, e.g., one or more of a-olefins such as ethylene,butene-1, pentene-1, hexene-1,4-methylpentene-1, octene-1, vinylacetate, methyl methacrylate, and styrene, etc.; and diolefins such asbutadiene, 1,5-hexadiene, 1,7-octadiene, 1,9-decadiene, etc. The amountof the fifth olefin in the copolymer is preferably in a range that doesnot adversely affect properties of the multi-layer microporous membranesuch as heat resistance, compression resistance, heat shrinkageresistance, etc. For example, the amount of the fifth olefin can be lessthan 10% by mol based on 100% by mol of the entire copolymer.

H. The Second Polypropylene

The second microporous layer materials can optionally comprise a secondpolypropylene. The polypropylene can be, for example, one or more of (i)a propylene homopolymer or (ii) a copolymer of propylene and a fiftholefin. The copolymer can be a random or block copolymer. The fiftholefin can be, e.g., one or more of a-olefins such as ethylene,butene-1, pentene-1, hexene-1,4-methylpentene-1, octene-1, vinylacetate, methyl methacrylate, and styrene, etc.; and diolefins such asbutadiene, 1,5-hexadiene, 1,7-octadiene, 1,9-decadiene, etc. The amountof the fifth olefin in the copolymer is preferably in a range that doesnot adversely affect properties of the multi-layer microporous membranesuch as heat resistance, compression resistance, heat shrinkageresistance, etc. For example, the amount of the fifth olefin can be lessthan 10% by mol based on 100% by mol of the entire copolymer. The secondpolypropylene can be the same as the first polyethylene, but this is notrequired.

I. Seventh Polyolefin

In addition to the first, second, third, and fourth polyethylenes andthe first and second polypropylenes, each of the first and second layermaterials can optionally contain one or more additional polyolefins,identified as the seventh polyolefin, which can be, e.g., one or more ofpolybutene-1, polypentene-1, poly-4-methylpentene-1, polyhexene-1,polyoctene-1, polyvinyl acetate, polymethyl methacrylate, polystyreneand an ethylene a-olefin copolymer (except for an ethylene-propylenecopolymer). In an embodiment where a seventh polyolefin is present, theseventh polyolefin can, for example, have an Mw in the range of about1×10⁴ to about 4×10⁶. In addition to or besides the seventh polyolefin,the first and second microporous layer materials can further comprise apolyethylene wax, e.g., one having an Mw in the range of about 1×10³ toabout 1×10⁴. When used, these species should be present in amounts lessthan an amount that would cause deterioration in the desired properties(e.g., meltdown, shutdown, etc.) of the multi-layer, microporousmembrane. When the seventh polyolefin is one or more of polybutene-1,polypentene-1, poly-4-methylpentene-1, polyhexene-1, polyoctene-1,polyvinyl acetate, polymethyl methacrylate, and polystyrene, the seventhpolyolefin need not be a homopolymer, but may be a copolymer containingother a-olefins.

[2] Composition of Materials Used to Produce the Multi-Layer,Microporous Polyolefin Membrane

In an embodiment, the first microporous layer material is produced froma first polyolefin solution. The first polyolefin solution comprises afirst polyolefin composition and a first membrane-forming solvent. Thefirst polyolefin composition is produced from a first resin or resins.Similarly, the second polyolefin solution comprises a second polyolefincomposition and a second membrane-forming solvent. The second polyolefincomposition is produced from a second resin or resins. These will now bedescribed in more detail.

A. First Polyolefin Composition

The first polyolefin composition is produced from at least onepolyethylene, e.g., in the form of one or more polyethylene resins. Inan embodiment, the first polyolefin composition can be made from a firstresin, which can a mixture of resins. For example, the first polyolefincomposition is made from a first resin, where the first resin comprisesresins of polyethylene only, or resins of polyethylene andpolypropylene. The first polyolefin composition can be made byconventional methods, e.g., by dry mixing or melt blending the resin(s).

1. First Resin

The first resin can be a mixture of resins. In an embodiment the firstresin is selected from resins of the following:

(a) a first polyethylene having an Mw that is less than 1×106;

(b) the first polyethylene and a second polyethylene having a Mw of atleast about 1×10⁶, wherein the second polyethylene is present in anamount that does not exceed about 7% by mass based on the combined massof the first and second polyethylene;

(c) the first polyethylene and a first polypropylene, wherein the amountof the polypropylene ranges does not exceed about 25% by mass based onthe combined mass of the first polyethylene and the first polypropylene;or

(d) the first polyethylene, the second polyethylene, and the firstpolypropylene, wherein the first polypropylene is present in an amountthat does not exceed about 25% by mass based on the combined mass of thefirst polyethylene, the second polyethylene, and the firstpolypropylene, and wherein the second polyethylene is present in anamount that does not exceed about 7% by mass based on the combined massof the first and second polyethylene.

In an embodiment, the first resin contains polyethylene only. In otherwords, in an embodiment, the first resin consists essentially ofpolyethylene or consists of polyethylene.

(1) Polyethylene Resins Used to Make the First Polyolefin Composition

In an embodiment, the first and second polyethylenes in the first resinare generally the same as the first and second polyethylenes describedabove in section [1]. The second polyethylene is present in the firstresin in an amount that does not exceed about 7% based on the totalamount of polyethylene in the first resin. When the amount the secondpolyethylene in the first resin is more than 7% by mass, it can be moredifficult to produce a multi-layer microporous polyolefin membraneexhibiting a hybrid structure in the first microporous layer material.The amount of the second polyethylene is preferably 5% or less by mass,or 3% or less by mass. The remainder of the first resin can be, e.g.,resins of the first polyethylene and/or first polypropylene.

The Mw of the polyethylene in first resin is not critical, and canrange, e.g., from about 1×10⁶ or less, or from about 1×10⁵ to about1×10⁶, or about 2×10⁵ to about 1×10⁶. When the Mw of the polyethylene inthe first polyolefin composition is more than about 1×10⁶, it can bemore difficult to produce a multi-layer microporous polyolefin membranehaving a first layer material characterized by a hybrid structure. Whenthe Mw of the first polyethylene composition is less than about 1×10⁵,it can be more difficult to produce a multi-layer microporous polyolefinmembrane that can be stretched without breaking or tearing.

(2) Polypropylene Resins Used to Make the First Polyolefin Composition

When the first polyolefin composition is produced from resins ofpolyethylene and resins of polypropylene, the amount of polypropylene inthe first polyolefin composition generally does not exceed about 25% bymass based on 100% of the mass of the first polyolefin composition. Whenthe amount of polypropylene exceeds about 25% by mass, it can be moredifficult to produce a multi-layer, microporous polyolefin membrane ofsufficient mechanical strength. The amount of polypropylene in the firstpolyolefin composition can be, e.g., about 15% or less by mass, or about10% or less by mass.

The type of the polypropylene in the first resin is not critical, andcan be a propylene homopolymer or a copolymer of propylene and a thirda-olefin and/or diolefin, or a mixture thereof. For example, in oneembodiment, the polypropylene is a homopolymer. When the polypropyleneis a copolymer, the copolymer can be a random or block copolymer. Thethird olefin is not propylene, and can be, e.g., one or more ofethylene, butene-1, pentene-1, hexene-1,4-methylpentene-1, octene-1,vinyl acetate, methyl methacrylate, styrene, etc., and diolefins such asbutadiene, 1,5-hexadiene, 1,7-octadiene, 1,9-decadiene, etc. The amountof the third olefin is not critical, and can be any amount that does notcause a deterioration in a desirable property of polypropylene such asheat resistance, compression resistance, heat shrinkage resistance. Inan embodiment, the amount of the third olefin is less than 10% by molbased on 100% by mol of the polypropylene copolymer.

The Mw of the polypropylene is not critical. In an embodiment, the Mwranges from about 1×10⁴ to about 4×10⁶, or about 3×10⁵ to about 3×106.The molecular weight distribution (Mw/Mn) of polypropylene is notcritical, and can range e.g., from about 1.01 to about 100, or about 1.1to about 50.

B. Second Polyolefin Composition

The second polyolefin composition is produced from at least onepolyethylene, e.g., in the form of one or more polyethylene resins. Inan embodiment, the second polyolefin composition can be made from asecond resin or mixture of resins. For example, the second polyolefincomposition is made from a second resin, where the second resincomprises resins of polyethylene only, or resins of polyethylene andpolypropylene. The second polyolefin composition can be made byconventional methods, e.g., by dry mixing or melt blending the resin(s).

1. Second Resin

Like the first resin, the second resin can be a mixture of resins. In anembodiment the second resin is selected from resins of the following:

-   -   (a) a fourth polyethylene having an Mw of at least about 1×106;    -   (b) a third polyethylene having an Mw that is less than 1×106        and the fourth polyethylene, wherein the fourth polyethylene is        present in an amount of at least about 8% by mass based on the        combined mass of the third and fourth polyethylene;    -   (c) the fourth polyethylene and a second polypropylene wherein        the second polypropylene is present in an amount that does not        exceed about 25% by mass based on the combined mass of the        fourth polyethylene and the second polypropylene; or    -   (d) the third polyethylene, the fourth polyethylene, and the        second polypropylene, wherein second polypropylene is present in        an amount that does not exceed about 25% by mass based on the        combined mass of the third polyethylene, the fourth        polyethylene, and the second polypropylene, and the fourth        polyethylene is present in an amount of at least about 8% by        mass based on the combined mass of the third and fourth        polyethylene.

In an embodiment, the second resin contains polyethylene only. In otherwords, in an embodiment, the second resin consists essentially ofpolyethylene or consists of polyethylene.

(1) Polyethylene Resins Used to Make the Second Polyolefin Composition

In an embodiment, the third and fourth polyethylenes in the second resinare generally the same as the first and second polyethylenes describedabove in section[1]. When the amount of the fourth polyethylene in thesecond resin is less than about 8% by mass based on the mass of thesecond resin, it can be more difficult to produce a relatively strongmulti-layer microporous polyolefin membrane. In an embodiment, theamount of the fourth polyethylene is, e.g., at least about 20% by mass,or at least about 25% by mass based on the mass of the second resin.

The second polyolefin composition is produced from at least onepolyethylene, e.g., in the form of one or more polyethylene resins. Inan embodiment, the second polyolefin composition can be made from asecond resin or mixture of resins. For example, the second polyolefincomposition is made from a second resin, where the second resincomprises resins of polyethylene only, or resins of polyethylene andpolypropylene. The second polyolefin composition can be made byconventional methods, e.g., by dry mixing or melt blending the resin(s).

(2) Polypropylene Resins Used to Make the Second Polyolefin Composition

When the second polyolefin composition is produced from resins ofpolyethylene and resins of polypropylene, the amount of polypropylene inthe second polyolefin composition generally does not exceed about 25% bymass based on 100% of the mass of the second polyolefin composition.When the amount of polypropylene exceeds about 25% by mass, it can bemore difficult to produce a multi-layer, microporous polyolefin membraneof sufficient mechanical strength. The amount of polypropylene in thesecond polyolefin composition can be, e.g., about 15% or less by mass,or about 10% or less by mass.

The type of the polypropylene in the second resin is not critical, andcan be a propylene homopolymer or a copolymer of propylene and a thirda-olefin and/or diolefin, or a mixture thereof. For example, in oneembodiment, the polypropylene is a homopolymer. When the polypropyleneis a copolymer, the copolymer can be a random or block copolymer. Thethird olefin is not propylene, and can be, e.g., one or more ofethylene, butene-1, pentene-1, hexene-1,4-methylpentene-1, octene-1,vinyl acetate, methyl methacrylate, styrene, etc., and diolefins such asbutadiene, 1,5-hexadiene, 1,7-octadiene, 1,9-decadiene, etc. The amountof the third olefin is not critical, and can be any amount that does notcause a deterioration in a desirable property of polypropylene such asheat resistance, compression resistance, heat shrinkage resistance. Inan embodiment, the amount of the third olefin is less than 10% by molbased on 100% by mol of the polypropylene copolymer.

The Mw of the polypropylene is not critical. In an embodiment, the Mwranges from about 1×10⁴ to about 4×10⁶, or about 3×10⁵ to about 3×106.

The molecular weight distribution (Mw/Mn) of polypropylene is notcritical, and can range e.g., from about 1.01 to about 100, or about 1.1to about 50.

C. Molecular Weight Distributions (Mw/Mn) of the Polyethylene in theFirst and Second Polyolefin Compositions

Mw/Mn is a measure of a molecular weight distribution; the larger thisvalue, the wider the molecular weight distribution. The molecular weightdistribution of the first polyolefin composition need not be the same asthe molecular weight distribution of the second polyolefin composition.Though not critical, Mw/Mn of the polyethylene in the first polyolefincomposition, the second polyolefin composition, or both can range, e.g.,from about 5 to about 300, or from about 5 to about 100, or about 5 toabout 30. When the Mw/Mn is less than about 5, it can be more difficultto extrude the polyolefin solution. On the other hand, when the Mw/Mn ismore than 300, it can be more difficult to produce a multi-layer,microporous polyolefin membrane of sufficient strength. The Mw/Mn ofpolyethylene (homopolymer or an ethylene a-olefin copolymer) can becontrolled by conventional methods, e.g., a multi-stage polymerization.The multi-stage polymerization method can be a two-stage polymerizationmethod comprising forming a relatively high molecular-weight polymercomponent in the first stage, and forming a relatively lowmolecular-weight polymer component in the second stage. In the casewhere the first and/or second polyolefin compositions contain the firstand second polyethylene, the larger the Mw/Mn, the larger difference inMw exists between the first polyethylene and the second polyethylene,and vice versa. In this case, the Mw/Mn of the polyethylene compositioncan be controlled, e.g., by controlling the relative molecular weightsand mixing ratios of the first and second polyethylene.

D. Other Components

In addition to the above components, the first and second polyolefincomposition can contain the other polyolefins besides the first throughfourth polyethylenes and the first and second polypropylene. Forexample, the first and second polyolefin compositions can contain otherpolyolefins and/or heat-resistant polymers having melting points orglass transition temperatures (Tg) of 170° C. or higher. The amount ofsuch species in the polyolefin composition is not critical and can be,e.g., any amount that does not significantly deteriorate the desiredproperties of the microporous polyolefin membrane.

(1) Other Polyolefins

The other polyolefins can be one or more of (a) polybutene-1,polypentene-1, poly-4-methylpentene-1, polyhexene-1, polyoctene-1,polyvinyl acetate, polymethyl methacrylate, polystyrene and an ethyleneα-olefin copolymer, each of which may have Mw of 1×10⁴ to 4×10⁶, and/or(b) a polyethylene wax having Mw of 1×10³ to 1×10⁴. Polybutene-1,polypentene-1, poly-4-methylpentene-1, polyhexene-1, polyoctene-1,polyvinyl acetate, polymethyl methacrylate and polystyrene are notrestricted to homopolymers, but can be copolymers containing othera-olefins.

(2) Heat-Resistant Resins

The type of heat-resistant resin, when one is used, is not critical. Theheat-resistant resins can be, e.g., crystalline resins having meltingpoints of 170° C. or higher, which may be partially crystalline, andamorphous resins having Tg of 170° C. or higher. The melting point andTg can be determined by, e.g., differential scanning calorimetry (DSC)according to JIS K7121. Specific examples of the heat-resistant resinsinclude polyesters such as polybutylene terephthalate (melting point:about 160 to 230° C.), polyethylene terephthalate (melting point: about250 to 270° C.), etc., fluororesins, polyamides (melting point: 215 to265° C.), polyarylene sulfide, polyimides (Tg: 280° C. or higher),polyamideimides (Tg: 280° C.), polyether sulfone (Tg: 223° C.),polyetheretherketone (melting point: 334° C.), polycarbonates (meltingpoint: 220 to 240° C.), cellulose acetate (melting point: 220° C.),cellulose triacetate (melting point: 300° C.), polysulfone (Tg: 190°C.), polyetherimide (melting point: 216° C.), etc.

The multi-layer microporous membrane generally comprises the materialsused to form the first and second polyolefin compositions. A smallamount of washing solvent and/or membrane-forming solvent can also bepresent in the membrane, generally in amounts less than 1 wt % based onthe weight of the microporous polyolefin membrane. A small amount ofmolecular weight degradation might occur during processing of thepolyolefins, but this is acceptable. In an embodiment, molecular weightdegradation during processing, if any, causes the value of Mw/Mn of thepolyolefin in the first or second layer material to differ from theMw/Mn of the first or second polyolefin compositions by no more thanabout 10%, or no more than about 1%, or no more than about 0.1%.

[3] Production Method of Multi-Layer, Microporous 5 Polyolefin Membrane

In an embodiment, the microporous polyolefin membrane is a two-layermembrane. In another embodiment, the microporous polyolefin membrane hasat least three layers. For the sake of brevity, the production of themicroporous polyolefin membrane will be mainly described in terms oftwo-layer and three-layer membrane, although those skilled in the artwill recognize that the same techniques can be applied to the productionof membranes or membranes having at least four layers.

In an embodiment, the three-layer microporous polyolefin membranecomprises first and third microporous layers constituting the outerlayers of the microporous polyolefin membrane and a second layersituated between (and optionally in planar contact with) the first andthird layers. In an embodiment, the first and third layers are producedfrom the first polyolefin solution and the second (or inner) layer isproduced from the second polyolefin solution.

A. First Production Method

The first method for producing the multi-layer, microporous polyolefinmembrane comprises the steps of (1) combining (e.g., by melt-blending) afirst polyolefin composition and a membrane-forming solvent to prepare afirst polyolefin solution, (2) combining a second polyolefin compositionand a second membrane-forming solvent to prepare a second polyolefinsolution, (3) extruding (preferably simultaneously) the first and secondpolyolefin solutions through at least one die to form an extrudate, (4)cooling the extrudate to form a multi-layer, gel-like sheet, (5)removing the membrane-forming solvent from the multi-layer, gel-likesheet to form a solvent-removed gel-like sheet, and (6) drying thesolvent-removed gel-like sheet in order to form the multi-layer,microporous polyolefin membrane. An optional stretching step (7), and anoptional hot solvent treatment step (8), etc. can be conducted betweensteps (4) and (5), if desired. After step (6), an optional step (9) ofstretching a multi-layer, microporous membrane, an optional heattreatment step (10), an optional cross-linking step with ionizingradiations (11), and an optional hydrophilic treatment step (12), etc.,can be conducted if desired. The order of the optional steps is notcritical.

Step (1) Preparation of First Polyolefin Solution

The first polyolefin composition comprises polyolefin resins asdescribed above that can be combined, e.g., by dry mixing or meltblending with an appropriate membrane-forming solvent to produce thefirst polyolefin solution. Optionally, the first polyolefin solution cancontain various additives such as one or more antioxidant, fine silicatepowder (pore-forming material), etc., provided these are used in aconcentration range that does not significantly degrade the desiredproperties of the multi-layer, microporous polyolefin membrane.

The first membrane-forming solvent is preferably a solvent that isliquid at room temperature. While not wishing to be bound by any theoryor model, it is believed that the use of a liquid solvent to form thefirst polyolefin solution makes it possible to conduct stretching of thegel-like sheet at a relatively high stretching magnification. In anembodiment, the first membrane-forming solvent can be at least one ofaliphatic, alicyclic or aromatic hydrocarbons such as nonane, decane,decalin, p-xylene, undecane, dodecane, liquid paraffin, etc.; mineraloil distillates having boiling points comparable to those of the abovehydrocarbons; and phthalates liquid at room temperature such as dibutylphthalate, dioctyl phthalate, etc. In an embodiment where it is desiredto obtain a multi-layer, gel-like sheet having a stable liquid solventcontent, non-volatile liquid solvents such as liquid paraffin can beused, either alone or in combination with other solvents. Optionally, asolvent which is miscible with polyethylene in a melt blended state butsolid at room temperature can be used, either alone or in combinationwith a liquid solvent. Such solid solvent can include, e.g., stearylalcohol, ceryl alcohol, paraffin waxes, etc. Although it is notcritical, it can be more difficult to evenly stretch the gel-like sheetor resulting membrane when the solution contains no liquid solvent.

The viscosity of the liquid solvent is not a critical parameter. Forexample, the viscosity of the liquid solvent can range from about 30 cStto about 500 cSt, or from about 30 cSt to about 200 cSt, at 25° C.Although it is not a critical parameter, when the viscosity at 25° C. isless than about 30 cSt, it can be more difficult to prevent foaming thepolyolefin solution, which can lead to difficulty in blending. On theother hand, when the viscosity is greater than about 500 cSt, it can bemore difficult to remove the liquid solvent from the multi-layermicroporous polyolefin membrane.

In an embodiment, the resins, etc., used to produce to the firstpolyolefin composition are melt-blended in, e.g., a double screwextruder or mixer. For example, a conventional extruder (or mixer ormixer-extruder) such as a double-screw extruder can be used to combinethe resins, etc., to form the first polyolefin composition. Themembrane-forming solvent can be added to the polyolefin composition (oralternatively to the resins used to produce the polyolefin composition)at any convenient point in the process. For example, in an embodimentwhere the first polyolefin composition and the first membrane-formingsolvent are melt-blended, the solvent can be added to the polyolefincomposition (or its components) at any of (i) before startingmelt-blending, (ii) during melt blending of the first polyolefincomposition, or (iii) after melt-blending, e.g., by supplying the firstmembrane-forming solvent to the melt-blended or partially melt-blendedpolyolefin composition in a second extruder or extruder zone locateddownstream of the extruder zone used to melt-blend the polyolefincomposition.

When melt-blending is used, the melt-blending temperature is notcritical. For example, the melt-blending temperature of the firstpolyolefin solution can range from about 10° C. higher than the meltingpoint Tm₁ of the polyethylene in the first resin to about 120° C. higherthan Tm₁. For brevity, such a range can be represented as Tm₁+10° C. toTin+120° C. In an embodiment where the polyethylene in the first resinhas a melting point of about 130° C. to about 140° C., the melt-blendingtemperature can range from about 140° C. to about 250° C., or from about170° C. to about 240° C.

When an extruder such as a double-screw extruder is used formelt-blending, the screw parameters are not critical. For example, thescrew can be characterized by a ratio L/D of the screw length L to thescrew diameter D in the double-screw extruder, which can range, forexample, from about 20 to about 100, or from about 35 to about 70.Although this parameter is not critical, when L/D is less than about 20,melt-blending can be more difficult, and when L/D is more than about100, faster extruder speeds might be needed to prevent excessiveresidence time of the polyolefin solution in the double-screw extruder(which can lead to undesirable molecular weight degradation). Althoughit is not a critical parameter, the cylinder (or bore) of thedouble-screw extruder can have an inner diameter of in the range ofabout 40 mm to about 100 mm, for example.

The amount of the first polyolefin composition in the first polyolefinsolution is not critical. In an embodiment, the amount of firstpolyolefin composition in the first polyolefin solution can range fromabout 1 wt. % to about 75 wt. %, based on the weight of the polyolefinsolution, for example from about 20 wt. % to about 70 wt. %.

When a hybrid structure is desired in the first microporous layermaterial, when the amount of first polyolefin composition in the firstpolyolefin solution ranges from about 25 wt. % to about 50 wt. %, it isless difficult to form a hybrid structure in the first microporous layermaterial. Optionally, the amount of first polyolefin composition in thefirst polyolefin solution ranges from about 25 wt. % to about 40 wt. %,based on the weight of the first polyolefin solution.

Although the amount of first polyolefin composition in the firstpolyolefin solution is not critical, when the amount is less than about1 wt. %, it can be more difficult to produce the multi-layer microporouspolyolefin membrane at an acceptably efficient rate. Moreover, when theamount is less than 1 wt. %, it can be more difficult to preventswelling or neck-in at the die exit during extrusion, which can make itmore difficult to form and support the multi-layer, gel-like sheet,which is a precursor of the membrane formed during the manufacturingprocess. On the other hand, when the amount of first polyolefincomposition solution is greater than about 75 wt. %, it can be moredifficult to form the multi-layer, gel-like sheet.

Step (2) Preparation of Second Polyolefin Solution

The second polyolefin solution can be prepared by the same methods usedto prepare the first polyolefin solution. For example, the secondpolyolefin solution can be prepared by melt-blending a second polyolefincomposition with a second membrane-forming solvent. The secondmembrane-forming solvent can be selected from among the same solvents asthe first membrane-forming solvent. And while the secondmembrane-forming solvent can be (and generally is) selectedindependently of the first membrane-forming solvent, the secondmembrane-forming solvent can be the same as the first membrane-formingsolvent, and can be used in the same relative concentration as the firstmembrane-forming solvent is used in the first polyolefin solution.

The second polyolefin composition is generally selected independently ofthe first polyolefin composition. The second polyolefin composition isproduced from the second resin. The second resin contains resins of thefourth polyethylene, and optionally, resins of the third polyethyleneand second polypropylene. Since the first and second microporous layermaterials generally do not have the same composition, the resins (andtheir relative amounts) used to produce the second polyolefincomposition can be and generally are different from the resins (andrelative amounts) used to produce the first polyolefin composition.

Although it is not a critical parameter, the melt-blending conditionsfor the second polyolefin solution can differ from the conditionsdescribed for producing the first polyolefin composition in that themelt-blending temperature of the second polyolefin solution can rangefrom about the melting point Tm₂ of the polyethylene in the secondresin+10° C. to Tm₂+120° C.

The amount of the second polyolefin composition in the second polyolefinsolution is not critical. In an embodiment, the amount of secondpolyolefin composition in the second polyolefin solution can range fromabout 1 wt. % to about 75 wt. %, based on the weight of the secondpolyolefin solution, for example from about 20 wt. % to about 70 wt. %.

Step (3) Extrusion

In an embodiment, the first polyolefin solution is conducted from afirst extruder to a first die and the second polyolefin solution isconducted from a second extruder to a second die. A layered extrudate insheet form (i.e., a body significantly larger in the lateral directionsthan in the thickness direction) can be extruded from the first andsecond die. Optionally, the first and second polyolefin solutions areco-extruded (e.g., simultaneously) from the first and second die with aplanar surface of a first extrudate layer formed from the firstpolyolefin solution in contact with a planar surface of a secondextrudate layer formed from the second polyolefin solution. A planarsurface of the extrudate can be defined by a first vector in the machinedirection of the extrudate and a second vector in the transversedirection of the extrudate.

In an embodiment, a die assembly is used where the die assemblycomprises the first and second die, as for example when the first dieand the second die share a common partition between a region in the dieassembly containing the first polyolefin solution and a second region inthe die assembly containing the second polyolefin solution.

In another embodiment, a plurality of dies is used, with each dieconnected to an extruder for conducting either the first or secondpolyolefin solution to the die. For example, in one embodiment, thefirst extruder containing the first polyolefin solution is connected toa first die and a third die, and a second extruder containing the secondpolyolefin solution is connected to a second die. As is the case in thepreceding embodiment, the resulting layered extrudate can be co-extrudedfrom the first, second, and third die (e.g., simultaneously) to form athree-layer extrudate comprising a first and a third layer constitutingsurface layers (e.g., top and bottom layers) produced from the firstpolyolefin solution; and a second layer constituting a middle orintermediate layer of the extrudate situated between and in planarcontact with both surface layers, where the second layer is producedfrom the second polyolefin solution.

In any of the preceding embodiments, die extrusion can be conductedusing conventional die extrusion equipment. For example, extrusion canbe conducted by a flat die or an inflation die. In one embodiment usefulfor co-extrusion of multi-layer gel-like sheets, multi-manifoldextrusion can be used, in which the first and second polyolefinsolutions are conducted to separate manifolds in a multi-layer extrusiondie and laminated at a die lip inlet. In another such embodiment, blockextrusion can be used, in which the first and second polyolefinsolutions are first combined into a laminar flow (i.e., in advance),with the laminar flow then connected to a die. Because multi-manifoldand block processes are known to those skilled in the art of processingpolyolefin films (e.g., as disclosed in JP06-122142 A, JP06-106599A),they are deemed conventional, therefore, their operation will be notdescribed in detail.

Die selection is not critical, and, e.g., a conventionalmulti-layer-sheet-forming, flat, tubular, or inflation die can be used.Die gap is not critical. For example, the multi-layer-sheet-forming flatdie can have a die gap of about 0.1 mm to about 5 mm. Die temperatureand extruding speed are also non-critical parameters. For example, thedie can be heated to a die temperature ranging from about 140° C. toabout 250° C. during extrusion. The extruding speed can range, forexample, from about 0.2 m/minute to about 15 m/minute. The thickness ofthe layers of the layered extrudate can be independently selected. Forexample, the gel-like sheet can have relatively thick surface layers (or“skin” layers) compared to the thickness of an intermediate layer of thelayered extrudate. In an embodiment, where the multi-layer, the layeredextrudate is a two-layer, the thickness ratio of the surface layer ofthe layered extrudate can range, e.g., from about 15% to about 60% basedon the total thickness of the layered extrudate, or from about 15% toabout 50%. In an embodiment, the thickness of the surface layer (firstlayer) of the layered extrudate can range, e.g., from about 15 μm toabout 3,000 μm, or from about 50 μm to about 2,000 μm. In an embodiment,the thickness of the other surface layer (second layer) of the layeredextrudate can range, e.g., from about 40 μm to about 4,000 μm, or fromabout 100 μm to about 2,000 μm. In an embodiment where the multi-layer,the layered extrudate is a three-layer, the thickness ratio of thelayers expressed as (surface layer/intermediate layer/surface layer) canrange, e.g., from about 1/(0.015 to 0.95)/1, or from about 1/(0.02 to0.8)/1, with the thickness of the surface layers normalized to 1. In anembodiment, the thickness of the first and third layer (surface layers)of the layered extrudate can range, e.g., from about 40 μm to about2,450 μm, or from about 100 μm to about 2,000 μm. In an embodiment, thethickness of the intermediate layer (second layer) of the layeredextrudate can range, e.g., from about 1 μm to about 1,600 μm, or fromabout 20 μm to about 1,000 μm.

While the extrusion has been described in terms of embodiments producingtwo and three-layer extrudates, the extrusion step is not limitedthereto. For example, a plurality of dies and/or die assemblies can beused to produce multi-layer extrudates having four or more layers usingthe extrusion methods of the preceding embodiments. In such a layeredextrudate, each surface or intermediate layer can be produced usingeither the first polyolefin solution and/or the second polyolefinsolution.

Step (4) Formation of a Multi-layer, Gel-like Sheet

The multi-layer extrudate can be formed into a multi-layer, gel-likesheet by cooling, for example. Cooling rate and cooling temperature arenot particularly critical. For example, the multi-layer, gel-like sheetcan be cooled at a cooling rate of at least about 50° C./minute untilthe temperature of the multi-layer, gel-like sheet (the coolingtemperature) is approximately equal to the multi-layer, gel-like sheet'sgelatin temperature (or lower). In an embodiment, the extrudate iscooled to a temperature of about 25° C. or lower in order to form themulti-layer gel-like sheet. While not wishing to be bound by any theoryor model, it is believed that cooling the layered extrudate sets thepolyolefin micro-phases of the first and second polyolefin solutions forseparation by the membrane-forming solvent or solvents. It has beenobserved that in general a slower cooling rate (e.g., less than 50°C./minute) provides the multi-layer, gel-like sheet with largerpseudo-cell units, resulting in a coarser higher-order structure. On theother hand, a relatively faster cooling rate (e.g., 80° C./minute)results in denser cell units. Although it is not a critical parameter,when the cooling rate of the extrudate is less than 50° C./minute,increased polyolefin crystallinity in the layer can result, which canmake it more difficult to process the multi-layer, gel-like sheet insubsequent stretching steps. The choice of cooling method is notcritical. For example conventional sheet cooling methods can be used. Inan embodiment, the cooling method comprises contacting the layeredextrudate with a cooling medium such as cooling air, cooling water, etc.Alternatively, the extrudate can be cooled via contact with rollerscooled by a cooling medium, etc.

Step (5) Removal of the First and Second Membrane-forming Solvents

In an embodiment, the first and second membrane-forming solvents areremoved (or displaced) from the multi-layer gel-like sheet in order toform a solvent-removed gel-like sheet. A displacing (or “washing”)solvent can be used to remove (wash away, or displace) the first andsecond membrane-forming solvents. While not wishing to be bound by anytheory or model, it is believed that because the polyolefin phases inthe multi-layer gel-like sheet produced from the first polyolefinsolution and the second polyolefin solution (i.e., the first polyolefinand the second polyolefin) are separated from the membrane-formingsolvent phase, the removal of the membrane-forming solvent provides aporous membrane constituted by fibrils forming a fine three-dimensionalnetwork structure and having pores communicating three-dimensionally andirregularly. The choice of washing solvent is not critical provided itis capable of dissolving or displacing at least a portion of the firstand/or second membrane-forming solvent. Suitable washing solventsinclude, for instance, one or more of volatile solvents such assaturated hydrocarbons such as pentane, hexane, heptane, etc.;chlorinated hydrocarbons such as methylene chloride, carbontetrachloride, etc.; ethers such as diethyl ether, dioxane, etc.;ketones such as methyl ethyl ketone, etc.; linear fluorocarbons such astrifluoroethane, C₆F₁₄, C₇F₁₆, etc.; cyclic hydrofluorocarbons such asC₅H₃F₇, etc.; hydrofluoroethers such as C₄F₉OCH₃, C₄F₉OC₂H₅, etc.; andperfluoroethers such as C₄F₉OCF₃, C₄F₉OC₂F₅, etc.

The method for removing the membrane-forming solvent is not critical,and any method capable of removing a significant amount of solvent canbe used, including conventional solvent-removal methods. For example,the multi-layer, gel-like sheet can be washed by immersing the sheet inthe washing solvent and/or showering the sheet with the washing solvent.The amount of washing solvent used is not critical, and will generallydepend on the method selected for removal of the membrane-formingsolvent. For example, the amount of washing solvent used can range fromabout 300 to about 30,000 parts by mass, based on the mass of thegel-like sheet. While the amount of membrane-forming solvent removed isnot particularly critical, generally a higher quality (more porous)membrane will result when at least a major amount of first and secondmembrane-forming solvent is removed from the gel-like sheet. In anembodiment, the membrane-forming solvent is removed from the gel-likesheet (e.g., by washing) until the amount of the remainingmembrane-forming solvent in the multi-layer gel-like sheet becomes lessthan 1 wt. %, based on the weight of the gel-like sheet.

Step (6) Drying of the Solvent-removed Gel-like Sheet

In an embodiment, the solvent-removed multi-layer, gel-like sheetobtained by removing the membrane-forming solvent is dried in order toremove the washing solvent. Any method capable of removing the washingsolvent can be used, including conventional methods such as heat-drying,wind-drying (moving air), etc. The temperature of the gel-like sheetduring drying (i.e., drying temperature) is not critical. For example,the drying temperature can be equal to or lower than the crystaldispersion temperature Tcd. Tcd is the lower of the crystal dispersiontemperature Tcd, of the polyethylene in the first resin and the crystaldispersion temperature Tcd₂ of the polyethylene in the second resin. Forexample, the drying temperature can be at least 5° C. below the crystaldispersion temperature Tcd. The crystal dispersion temperature of thepolyethylene in the first and second resins can be determined bymeasuring the temperature characteristics of the kinetic viscoelasticityof the polyethylene according to ASTM D 4065. In an embodiment, thepolyethylene in at least one of the first or second resins has a crystaldispersion temperature in the range of about 90° C. to about 100° C.

Although it is not critical, drying can be conducted until the amount ofremaining washing solvent is about 5 wt. % or less on a dry basis, i.e.,based on the weight of the dry multi-layer, microporous polyolefinmembrane. In another embodiment, drying is conducted until the amount ofremaining washing solvent is about 3 wt. % or less on a dry basis.Insufficient drying can be recognized because it generally leads to anundesirable decrease in the porosity of the multi-layer, microporousmembrane. If this is observed, an increased drying temperature and/ordrying time should be used. Removal of the washing solvent, e.g., bydrying or otherwise, results in the formation of the multi-layer,microporous polyolefin membrane.

Step (7) Stretching

Prior to the step for removing the membrane-forming solvents (namelyprior to step 5), the multi-layer, gel-like sheet can be stretched inorder to obtain a stretched, multi-layer, gel-like sheet. It is believedthat the presence of the first and second membrane-forming solvents inthe multi-layer, gel-like sheet result in a relatively uniformstretching magnification. Heating the multi-layer, gel-like sheet,especially at the start of stretching or in a relatively early stage ofstretching (e.g., before 50% of the stretching has been completed) isalso believed to aid the uniformity of stretching.

Neither the choice of stretching method nor the degree of stretchingmagnification are particularly critical. For example, any method capableof stretching the multi-layer, gel-like sheet to a predeterminedmagnification (including any optional heating) can be used. In anembodiment, the stretching can be accomplished by one or more oftenter-stretching, roller-stretching, or inflation stretching (e.g.,with air). Although the choice is not critical, the stretching can beconducted monoaxially (i.e., in either the machine or transversedirection) or biaxially (both the machine or transverse direction). Inan embodiment, biaxial stretching is used. In the case of biaxialstretching (also called biaxial orientation), the stretching can besimultaneous biaxial stretching, sequential stretching along one planaraxis and then the other (e.g., first in the transverse direction andthen in the machine direction), or multi-stage stretching (for instance,a combination of the simultaneous biaxial stretching and the sequentialstretching). In an embodiment, simultaneous biaxial stretching is used.

The stretching magnification is not critical. In an embodiment wheremonoaxial stretching is used, the linear stretching magnification canbe, e.g., about 2 fold or more, or about 3 to about 30 fold. In anembodiment where biaxial stretching is used, the linear stretchingmagnification can be, e.g., about 3 fold or more in any lateraldirection. In another embodiment, the linear magnification resultingfrom stretching is at least about 9 fold, or at least about 16 fold, orat least about 25 fold in area magnification. Although it is not acritical parameter, when the stretching results in an area magnificationof at least about 9 fold, the multi-layer microporous polyolefinmembrane has a relatively higher pin puncture strength. When attemptingan area magnification of more than about 400 fold, it can be moredifficult to operate the stretching apparatus.

The temperature of the multi-layer, gel-like sheet during stretching(namely the stretching temperature) is not critical. In an embodiment,the temperature of the gel-like sheet during stretching can be about(Tm+10° C.) or lower, or optionally in a range that is higher than Tcdbut lower than Tm, wherein Tm is the lesser of the melting point Tm_(i)of the polyethylene in the first resin and the melting point Tm₂ of thepolyethylene in the second resin. Although this parameter is notcritical, when the stretching temperature is higher than approximatelythe melting point Tm+10° C., at least one of the first or second resinscan be in the molten state, which can make it more difficult to orientthe molecular chains of the polyolefin in the multi-layer gel-like sheetduring stretching. And when the stretching temperature is lower thanapproximately Tcd, at least one of the first or second resins can be soinsufficiently softened that it is difficult to stretch the multi-layer,gel-like sheet without breakage or tears, which can result in a failureto achieve the desired stretching magnification. In an embodiment, thestretching temperature ranges from about 90° C. to about 140° C., orfrom about 100° C. to about 130° C.

While not wishing to be bound by any theory or model, it is believedthat such stretching causes cleavage between polyethylene lamellas,making the polyethylene phases finer and forming large numbers offibrils. The fibrils form a three-dimensional network structure(three-dimensionally irregularly connected network structure).Consequently, the stretching when used generally makes it easier toproduce a relatively high-mechanical strength multi-layer, microporouspolyolefin membrane with a relatively large pore size. Such multi-layer,microporous membranes are believed to be particularly suitable for useas battery separators.

Optionally, stretching can be conducted in the presence of a temperaturegradient in a thickness direction (i.e., a direction approximatelyperpendicular to the planar surface of the multi-layer, microporouspolyolefin membrane). In this case, it can be easier to produce amulti-layer, microporous polyolefin membrane with improved mechanicalstrength. The details of this method are described in Japanese Patent3347854.

Step (8) Hot Solvent Treatment Step

Although it is not required, the multi-layer, gel-like sheet can betreated with a hot solvent between steps (4) and (5). When used, it isbelieved that the hot solvent treatment provides the fibrils (such asthose formed by stretching the multi-layer gel-like sheet) with arelatively thick leaf-vein-like structure. Such a structure, it isbelieved, makes it less difficult to produce a multi-layer, microporousmembrane having large pores with relatively high strength andpermeability. The term “leaf-vein-like” means that the fibrils havethick trunks and thin fibers extending therefrom in a network structure.The details of this method are described in WO 2000/20493.

Step (9) Stretching of multi-layer, microporous membrane (“drystretching”)

In an embodiment, the dried multi-layer, microporous membrane of step(6) can be stretched, at least monoaxially. The stretching methodselected is not critical, and conventional stretching methods can beused such as by a tenter method, etc. While it is not critical, themembrane can be heated during stretching. While the choice is notcritical, the stretching can be monoaxial or biaxial. When biaxialstretching is used, the stretching can be conducted simultaneously inboth axial directions, or, alternatively, the multi-layer, microporouspolyolefin membrane can be stretched sequentially, e.g., first in themachine direction and then in the transverse direction. In anembodiment, simultaneous biaxial stretching is used. When themulti-layer gel-like sheet has been stretched as described in step (7)the stretching of the dry multi-layer, microporous polyolefin membranein step (9) can be called dry-stretching, re-stretching, ordry-orientation.

The temperature of the dry multi-layer, microporous membrane duringstretching (the “dry stretching temperature”) is not critical. In anembodiment, the dry stretching temperature is approximately equal to themelting point Tm or lower, for example in the range of from about thecrystal dispersion temperature Tcd to the about the melting point Tm.When the dry stretching temperature is higher than Tm, it can be moredifficult to produce a multi-layer, microporous polyolefin membranehaving a relatively high compression resistance with relatively uniformair permeability characteristics, particularly in the transversedirection when the dry multi-layer, microporous polyolefin membrane isstretched transversely. When the stretching temperature is lower thanTcd, it can be more difficult to sufficiently soften the first andsecond polyolefins, which can lead to tearing during stretching, and alack of uniform stretching In an embodiment, the dry stretchingtemperature ranges from about 90° C. to about 135° C., or from about 95°C. to about 130° C.

When dry-stretching is used, the stretching magnification is notcritical. For example, the stretching magnification of the multi-layer,microporous membrane can range from about 1.1 fold to about 1.8 fold inat least one lateral (planar) direction. Thus, in the case of monoaxialstretching, the stretching magnification can range from about 1.1 foldto about 1.8 fold in the machine direction (i.e., the “machinedirection”) or the transverse direction, depending on whether themembrane is stretched longitudinally or transversely. Monoaxialstretching can also be accomplished along a planar axis between themachine and transverse directions.

In an embodiment, biaxial stretching is used (i.e., stretching along twoplanar axis) with a stretching magnification of about 1.1 fold to about1.8 fold along both stretching axes, e.g., along both the machine andtransverse directions. The stretching magnification in the machinedirection need not be the same as the stretching magnification in thetransverse direction. In other words, in biaxial stretching, thestretching magnifications can be selected independently. In anembodiment, the dry-stretching magnification is the same in bothstretching directions.

Step (10) Heat Treatment

In an embodiment, the dried multi-layer, microporous membrane can beheat-treated following step (6). It is believed that heat-treatingstabilizes the polyolefin crystals in the dried multi-layer, microporouspolyolefin membrane to form uniform lamellas. In an embodiment, the heattreatment comprises heat-setting and/or annealing. When heat-setting isused, it can be conducted using conventional methods such as tentermethods and/or roller methods. Although it is not critical, thetemperature of the dried multi-layer, microporous polyolefin membraneduring heat-setting (i.e., the “heat-setting temperature”) can rangefrom the Tcd to about the Tm. In an embodiment, the heat-settingtemperature ranges from about the dry stretching temperature of themulti-layer, microporous polyolefin membrane ±5° C., or about the drystretching temperature of the multi-layer, microporous polyolefinmembrane ±3° C.

Annealing differs from heat-setting in that it is a heat treatment withno load applied to the multi-layer, microporous polyolefin membrane. Thechoice of annealing method is not critical, and it can be conducted, forexample, by using a heating chamber with a belt conveyer or anair-floating-type heating chamber. Alternatively, the annealing can beconducted after the heat-setting with the tenter clips slackened. Thetemperature of the multi-layer, microporous polyolefin membrane duringannealing (i.e., the annealing temperature) is not critical. In anembodiment, the annealing temperature ranges from about the meltingpoint Tm or lower, or in a range from about 60° C. to (Tm-10° C.), or ina range of from about 60° C. to (Tm-5° C.). It is believed thatannealing makes it less difficult to produce a multi-layer, microporouspolyolefin membrane having relatively high permeability and strength.

Step (11) Cross-Linking

In an embodiment, the multi-layer, microporous polyolefin membrane canbe cross-linked (e.g., by ionizing radiation rays such as a-rays,(3-rays, y-rays, electron beams, etc.) after step (6). For example, whenirradiating electron beams are used for cross-linking, the amount ofelectron beam radiation can be about 0.1 Mrad to about 100 Mrad, usingan accelerating voltage in the range of about 100 kV to about 300 kV. Itis believed that the cross-linking treatment makes it less difficult toproduce a multi-layer, microporous polyolefin membrane with relativelyhigh meltdown temperature.

Step (12) Hydrophilizing Treatment

In an embodiment, the multi-layer, microporous polyolefin membrane canbe subjected to a hydrophilic treatment (i.e., a treatment which makesthe multi-layer, microporous polyolefin membrane more hydrophilic). Thehydrophilic treatment can be, for example, a monomer-grafting treatment,a surfactant treatment, a corona-discharging treatment, etc. In apreferable embodiment, the monomer-grafting treatment is used after thecross-linking treatment.

When a surfactant treatment is used, any of nonionic surfactants,cationic surfactants, anionic surfactants and amphoteric surfactants canbe used, for example, either alone or in combination. In an embodiment,a nonionic surfactant is used. The choice of surfactant is not critical.For example, the multi-layer, microporous polyolefin membrane can bedipped in a solution of the surfactant and water or a lower alcohol suchas methanol, ethanol, isopropyl alcohol, etc., or coated with thesolution, e.g., by a doctor blade method.

B. Second Production Method

The second method for producing the multi-layer, microporous polyolefinmembrane comprises the steps of (1) combining (e.g., by melt-blending) afirst polyolefin composition and a first membrane-forming solvent toprepare a first polyolefin solution, (2) combining a second polyolefincomposition and a second membrane-forming solvent to prepare a secondpolyolefin solution, (3) extruding the first polyolefin solution througha first die and the second solution through a second die and thenlaminating the extruded first and second polyolefin solutions to form amulti-layer extrudate, (4) cooling the multi-layer extrudate to form amulti-layer, gel-like sheet, (5) removing the membrane-forming solventfrom the multi-layer, gel-like sheet to form a solvent-removed gel-likesheet, and (6) drying the solvent-removed gel-like sheet in order toform the multi-layer, microporous membrane. An optional stretching step(7), and an optional hot solvent treatment step (8), etc., can beconducted between steps (4) and (5), if desired. After step (6), anoptional step (9) of stretching a multi-layer, microporous membrane, anoptional heat treatment step (10), an optional cross-linking step withionizing radiations (11), and an optional hydrophilic treatment step(12), etc., can be conducted.

The process steps and conditions of the second production method aregenerally the same as those of the analogous steps described inconnection with the first production method, except for step (3).Consequently, step (3) will be explained in more detail.

The type of die used is not critical provided the die is capable offorming an extrudate that can be laminated. In one embodiment, sheetdies (which can be adjacent or connected) are used to form theextrudates. The first and second sheet dies are connected to first andsecond extruders, respectively, where the first extruder contains thefirst polyolefin solution and the second extruder contains the secondpolyolefin solution. While not critical, lamination is generally easierto accomplish when the extruded first and second polyolefin solution arestill at approximately the extrusion temperature. The other conditionsmay be the same as in the first method.

In another embodiment, the first, second, and third sheet dies areconnected to first, second and third extruders, where the first andthird sheet dies contain the first polyolefin solutions, and the secondsheet die contains the second polyolefin solution. In this embodiment, alaminated extrudate is formed constituting outer layers comprising theextruded first polyolefin solution and one intermediate comprising theextruded second polyolefin solution.

In yet another embodiment, the first, second, and third sheet dies areconnected to first, second, and third extruders, where the second sheetdie contains the first polyolefin solution, and the first and thirdsheet dies contain the second polyolefin solution. In this embodiment, alaminated extrudate is formed constituting outer layers comprising theextruded second polyolefin solution and one intermediate comprisingextruded first polyolefin solution.

C. Third Production Method

The third method for producing the multi-layer, microporous polyolefinmembrane comprises the steps of (1) combining (e.g., by melt-blending) afirst polyolefin composition and a membrane-forming solvent to prepare afirst polyolefin solution, (2) combining a second polyolefin compositionand a second membrane-forming solvent to prepare a second polyolefinsolution, (3) extruding the first polyolefin solution through at leastone first die to form at least one first extrudate, (4) extruding thesecond polyolefin solution through at least one second die to form atleast one second extrudate, (5) cooling first and second extrudates toform at least one first gel-like sheet and at least one second gel-likesheet, (6) laminating the first and second gel-like sheet to form amulti-layer, gel-like sheet, (7) removing the membrane-forming solventfrom the resultant multi-layer, gel-like sheet to form a solvent-removedgel-like sheet, and (8) drying the solvent-removed gel-like sheet inorder to form the multi-layer, microporous membrane. An optionalstretching step (9), and an optional hot solvent treatment step (10),etc., can be conducted between steps (5) and (6) or between steps (6)and (7), if desired. After step (8), an optional step (11) of stretchinga multi-layer, microporous membrane, an optional heat treatment step(12), an optional cross-linking step with ionizing radiations (13), andan optional hydrophilic treatment step (14), etc., can be conducted.

The main difference between the third production method and the secondproduction method is in the order of the steps for laminating andcooling. In the second production method, laminating the first andsecond polyolefin solutions is conducted before the cooling step. In thethird production method, the first and second polyolefin solutions arecooled before the laminating step.

Steps (1), (2), (7) and (8) in the third production method can be thesame as the steps of (1), (2), (5) and (6) in the first productionmethod. For the extrusion of the first polyolefin solution through thefirst die, the conditions of step (3) of the second production methodcan be used for step (3) of the third production method. For theextrusion of the second solution through the second die, the conditionsof step (4) in the third production method can be the same as theconditions of step (3) in the second production method. In oneembodiment, either the first or second polyolefin solution is extrudedthrough a third die. In this way, a multi-layer laminate can be formedhaving two layers produced from the first polyolefin solution and asingle layer produced from the second polyolefin solution, or viceversa.

Step (5) of the third production method can be the same as step (4) inthe first production method except that in the third production methodthe first and second gel-like sheets are formed separately.

Step (6) of laminating the first and second gel-like sheets will now beexplained in more detail. The choice of lamination method is notparticularly critical, and conventional lamination methods such asheat-induced lamination can be used to laminate the multi-layer gel-likesheet. Other suitable lamination methods include, for example,heat-sealing, impulse-sealing, ultrasonic-bonding, etc., either alone orin combination. Heat-sealing can be conducted using, e.g., one or morepair of heated rollers where the gel-like sheets are conducted throughat least one pair of the heated rollers. Although the heat-sealingtemperature and pressure are not particularly critical, sufficientheating and pressure should be applied for a sufficient time to ensurethat the gel-like sheets are appropriately bonded to provide amulti-layer, microporous membrane with relatively uniform properties andlittle tendency toward delamination. In an embodiment, the heat-sealingtemperature can be, for instance, about 90° C. to about 135° C., or fromabout 90° C. to about 115° C. In an embodiment, the heat-sealingpressure can be from about 0.01 MPa to about 50 MPa.

As is the case in the first and second production method, the thicknessof the layers formed from the first and second polyolefin solution(i.e., the layers comprising the first and second microporous layermaterials) can be controlled by adjusting the thickness of the first andsecond gel-like sheets and by the amount of stretching (stretchingmagnification and dry stretching magnification), when one or morestretching steps are used. Optionally, the lamination step can becombined with a stretching step by passing the gel-like sheets throughmulti-stages of heated rollers.

In an embodiment, the third production method forms a multi-layer,polyolefin gel-like sheet having at least three layers. For example,after cooling two extruded first polyolefin solutions and one extrudedsecond polyolefin solution to form the gel-like sheets, the multi-layergel-like sheet can be laminated with outer layers comprising theextruded first polyolefin solution and an intermediate layer comprisingthe extruded second polyolefin solution. In another embodiment, aftercooling two extruded second polyolefin solutions and one extruded firstpolyolefin solution to form the gel-like sheets, the multi-layergel-like sheet can be laminated with outer layers comprising theextruded second polyolefin solution and an intermediate layer comprisingthe extruded first polyolefin solution.

The stretching step (9) and the hot solvent treatment step (10) can bethe same as the stretching step (7) and the hot solvent treatment step(8) as described for the first production method, except stretching step(9) and hot solvent treatment step (10) are conducted on the firstand/or second gel-like sheets. The stretching temperatures of the firstand second gel-like sheets are not critical. For example, the stretchingtemperatures of the first gel-like sheet can be, e.g., Tm₁+10° C. orlower, or optionally about Rd or higher but lower than about Tm₁. Thestretching temperature of the second gel-like sheet can be, e.g.,Tm₂+10° C. or lower, or optionally about Tcd₂ or higher but lower thanabout Tm₂.

In another embodiment, the stretching temperature of the first gel-likesheet ranges from about the crystal dispersion temperature Tcd₁ of thepolyethylene in the first resin to Tcd₁±25° C., or from about Tcd₁+10°C. to Tcd₁+25° C., or from about Tcd₁+15° C. to Tcd₁+25° C. Thestretching temperature of the second gel-like sheet ranges from thecrystal dispersion temperature Tcd₂ of the polyethylene in the secondresin to about Tcd₂+25° C., or about Tcd₂+10° C. to Tcd₂+25° C., orabout Tcd₂+15° C. to Tcd₂+25° C.

D. Fourth Production Method

The fourth method for producing the multi-layer, microporous polyolefinmembrane comprises the steps of (1) combining (e.g., by melt-blending) afirst polyolefin composition and a membrane-forming solvent to prepare afirst polyolefin solution, (2) combining a second polyolefin compositionand a second membrane-forming solvent to prepare a second polyolefinsolution, (3) extruding the first polyolefin solution through at leastone first die to form at least one first extrudate, (4) extruding thesecond polyolefin solution through at least one second die to form atleast one second extrudate, (5) cooling first and second extrudates toform at least one first gel-like sheet and at least one second gel-likesheet, (6) removing the first and second membrane-forming solvents fromthe first and second gel-like sheets to form solvent-removed first andsecond gel-like sheets, (7) drying the solvent-removed first and secondgel-like sheets to form at least one first polyolefin membrane and atleast one second polyolefin membrane, and (8) laminating the first andsecond microporous polyolefin membranes in order to form themulti-layer, microporous polyolefin membrane.

A stretching step (9), a hot solvent treatment step (10), etc., can beconducted between steps (5) and (6), if desired. A stretching step (11),a heat treatment step (12), etc., can be conducted between steps (7) and(8), if desired. After step (8), a step (13) of stretching amulti-layer, microporous membrane, a heat treatment step (14), across-linking step with ionizing radiations (15), a hydrophilictreatment step (16), etc., can be conducted if desired.

Steps (1) and (2) in the fourth production method can be conducted underthe same conditions as steps of (1) and (2) in the first productionmethod. Steps (3), (4), and (5) in the fourth production method can beconducted under the same conditions as steps (3), (4), and (5) in thethird method. Step (6) in the fourth production method can be conductedunder the same conditions as step (5) in the first production methodexcept for removing the membrane-forming solvent from the first andsecond gel-like sheets. Step (7) in the fourth production method can beconducted under the same conditions as step (6) in the first productionmethod except that in the fourth production method the first and secondsolvent-removed gel-like sheets are dried separately. Step (8) in thefourth production method can be conducted under the same conditions asthe step (6) in the third production method except for laminating thefirst and second polyolefin microporous membranes. The stretching step(9) and the hot solvent treatment step (10) in the fourth productionmethod can be conducted under the same conditions as step (9) and (10)in the third production method. The stretching step (11) and the heattreatment step (12) in the fourth production method can be conductedunder the same conditions as steps (9) and (10) in the first productionmethod except that in the fourth production method the first and secondpolyolefin microporous membranes are stretched and/or heat treated.

In an embodiment, in the stretching step (11) in the fourth productionmethod, the stretching temperature of the first microporous polyolefinmembrane can be about Tm₁ or lower, or optionally about Tcd₁ to aboutTm₁, and the stretching temperature of the second microporous polyolefinmembrane can be about Tm₂ or lower, or optionally about Tcd₂ to aboutTm₂.

In an embodiment, the heat treatment step (12) in the fourth productionmethod can be heat-setting and/or annealing. For example, in the heattreatment step (12) in the fourth production method, the heat-settingtemperature of the first polyolefin microporous membranes can be aboutTcd₁ to about Tm₁, or optionally about the dry stretching temperature±5° C., or optionally about the dry stretching temperature ±3° C. In anembodiment, in the heat treatment step (12) in the fourth productionmethod, the heat-setting temperature of the second microporous membranecan be about Tcd₂ to about Tm₂, or optionally the dry stretchingtemperature ±5° C., or optionally the dry stretching temperature ±3° C.When the heat-setting is used, it can be conducted by, e.g., a tentermethod or a roller method.

In an embodiment, the heat treatment step (12) in the fourth productionmethod can be heat-setting and/or annealing. For example, in the heattreatment step (12) in the fourth production method, the heat-settingtemperature of the first polyolefin microporous membranes can be aboutTcd₁ to about Tm₁, or optionally about the dry stretching temperature±5° C., or optionally about the dry stretching temperature ±3° C. In anembodiment, in the heat treatment step (12) in the fourth productionmethod, the heat-setting temperature of the second microporous membranecan be about Tcd₂ to about Tm₂, or optionally the dry stretchingtemperature ±5° C., or optionally the dry stretching temperature ±3° C.When the heat-setting is used, it can be conducted by, e.g., a tentermethod or a roller method.

The conditions in step (13), stretching a multi-layer, microporousmembrane, a heat treatment step (14), a cross-linking step with ionizingradiations (15), and a hydrophilic treatment step (16) in the fourthproduction method can be the same as those for steps (9), (10), (11) and(12) in the first production method.

[4] Structure, Properties, and Composition of Multi-Layer, MicroporousPolyolefin Membrane

In an embodiment, the multi-layer, microporous polyolefin membranecomprises a first (or top) layer, a third (or bottom) layer, and asecond (middle) layer located between the first and the third layer. Thefirst and third layers constitute the top and bottom surfacesrespectively of the membrane when the membrane is horizontal. Themulti-layer, microporous polyolefin membrane further comprises a secondlayer situated between the first and third layers, and optionally incontact with (e.g., adhered to) the first and/or third layer. Since thesecond layer is. situated between the first and third layers, it can bereferred to as a “middle” or “intermediate” layer. In an embodiment, themulti-layer, microporous polyolefin membrane further comprises aplurality of intermediate layers.

In an embodiment, the first and third layers of the multi-layer,microporous polyolefin membrane both comprise the same first microporouslayer material. The second layer of the multi-layer, microporouspolyolefin membrane comprises a second microporous layer material. Inanother embodiment, the first and third layers consist essentially of,or alternatively consist of, the first microporous layer material; andthe second layer consists of, or alternatively consists essentially of,the second microporous layer material. Additional intermediate layersbeside the second intermediate layer are optional. When present, theycan comprise, consist of, or consist essentially of the first or secondmicroporous layer material, as desired, or some other layer materialhaving desired strength, stability, electrolyte permeability properties,etc.

(A) Properties of the First Microporous Layer Material

(1) Average pore size

In an embodiment, the average pore size of the pores of firstmicroporous layer material ranges from 0.02 to 0.5 μm, preferably from0.02 to 0.1 μm. As used herein, the term “pore size” is analogous to thepore size in the case where the pores are approximately spherical.

(2) Structure

In an embodiment, the first microporous layer material has ischaracterized by a hybrid structure. A hybrid structure exists when thepore size distribution curve as obtained by mercury intrusionporosimetry has at least two peaks, a main peak in a pore size rangingfrom 0.01 to 0.08 μm, and at least one sub-peak in a pore size range ofgreater than 0.08 μm to 1.5 μm. See, e.g., FIG. 1. The main peakrepresents dense domains, and the sub-peaks represent coarse domains. Itis the presence of the relatively coarse domains in the first layermaterial that result in the pores of the first layer material having arelatively larger average pore size than the pores of the secondmicroporous layer material. It is believed that whether a hybridstructure forms depends mainly on polymer resins used to produce thefirst resin. For example, when the amount of ultra-high-molecular-weightpolyethylene in the first resin is more than 7% by mass based on themass of the first resin, it can be more difficult to form the desiredhybrid structure. The presence of a hybrid structure in the firstmicroporous layer material is generally desired because, it is believed,the hybrid structure leads to relatively high poor electrolytic solutionabsorption properties in the multi-layer, microporous polyolefinmembrane.

Optionally, the first microporous layer comprises dense domains having amain peak (first peak) in a pore size ranging from about 0.04 μm toabout 0.07 μm, and coarse domains having at least a second peak at apore size ranging from about 0.1 μm to about 0.11 μm, a third peak at apore size of about 0.7 μm, and a fourth peak at a pore size ranging fromabout 1 μm to 1.1 μm. FIG. 1 shows an example of the measured pore sizedistribution curve. In this example, the first to fourth peaks arelocated at about 0.06 μm, about 0.1 μm, about 0.7 μm, and about 1.1 μmrespectively.

The pore volume of the dense domains is calculated from the area of themain peak (first peak), and the pore volume of the coarse domains iscalculated from the total area of the sub-peaks (second to fourthpeaks). The pore volume ratio of the dense domains to the coarse domainsis determined by S₁ and S₂ shown in FIG. 1. A hatched area S_(i) on thesmaller diameter side than a vertical line L_(i) passing the first peakcorresponds to the pore volume of the dense domains, and a hatched areaS₂ on the larger diameter side than a vertical line L₂ passing thesecond peak corresponds to the pore volume of the coarse domains. Thepore volume ratio S₁/S₂ of the dense domains to the coarse domains isnot critical, and can range, e.g., from about 0.5 to about 49, or fromabout 0.6 to about 10, or about 0.7 to about 2.

Although it is not critical, the dense domains and the coarse domains inthe first microporous layer material can be irregularly entangled toform a hybrid structure in any cross” sections of the first microporouslayer as viewed in machine and transverse directions. The hybridstructure can be observed by a transmission electron microscope (TEM),etc.

(3) Number of Layers

In an embodiment, the first microporous layer material constitutes atleast both surface layers (i.e., the first and third layers) of athree-layer, microporous polyolefin membrane. In another embodiment, themulti-layer, microporous polyolefin membrane has more than three layers,with both surface layers, and optionally one or more of the interiorlayers, comprising the first microporous layer material.

(4) Function of First Microporous Layer

When the multi-layer, microporous polyolefin membrane is a two-layermembrane, one surface layer of the membrane comprises the firstmicroporous layer material and the second surface layer comprises thesecond microporous layer material. Since in this embodiment themulti-layer, microporous polyolefin membrane is a two-layer membrane,there are no interior layers. In this embodiment, the first surfacelayer has a larger average pore size than the second surface layer. Thefirst surface layer generally has a relatively high permeability,electrolytic solution absorption, and compression resistance compared tothe second layer.

When the multi-layer, microporous polyolefin membrane is a three-layermembrane, both surface layers of the membrane comprises the firstmicroporous layer material and the at least one interior layer comprisesthe second microporous layer material. In this embodiment, the first andsecond surface layers have a larger average pore size than the interiorlayer or layers comprising the second microporous layer material. Thesurface layers generally have a relatively high permeability,electrolytic solution absorption, and compression resistance compared tothe interior layer or layers comprising the second layer material.

(B) Properties of the Second Microporous Layer Material

(1) Average Pore Size

In an embodiment, the average pore size of the second microporous layermaterial ranges from about 0.005 μm to about 0.1 μm, or from about 0.01μm to about 0.05 μm.

(2) Number of Layers

In an embodiment, the second layer material constitutes one surface 25layer of a two-layer microporous polyolefin membrane, with the firstlayer material constituting the other surface layer.

In another embodiment, the multi-layer, microporous polyolefin membraneis a three-layer membrane, i.e., it consists of three layers—a firstlayer constituting a first surface layer of the membrane, a third layerconstituting a second surface layer of the membrane, and a second layerconstituting an interior layer located between the first and thirdlayers and in planar contact with the first and third layers. The secondlayer comprises the second layer material. Multi-layer, microporouspolyolefin membrane having more than three layers comprise a first layerconstituting a first surface layer of the membrane, a third layerconstituting a second surface layer of the membrane, and at least twointerior layers located between the first and third layers. The firstand third layers comprise the first layer material and at least one ofthe interior layers comprise the second layer material.

(3) Function of Second Microporous Layer

It is believed that when the multi-layer, microporous polyolefinmembranes has at least one layer comprising the first microporous layermaterial and at least one layer comprising the second microporous layermaterial, it is less difficult to produce a battery separator havingsuitably high permeability, electrolytic solution absorption andcompression resistance characteristics.

(C) Average Pore Size Ratio

In an embodiment, the average pore size ratio of the pores in the secondmicroporous layer material to the first microporous layer materialranges, e.g., from a value larger than 1/1 to about 10/1, or from about1.5/1 to about 5/1.

(D) Arrangement and Thickness Ratios of First, and, Second MicroporousLayer Materials

In an embodiment where the multi-layer, microporous polyolefin membraneis a two-layer membrane, the thickness of the first surface layer canrange, e.g., from 15% to 60% based on the total thickness of themulti-layer microporous polyolefin membrane, or from about 15% to 50%.In an embodiment where the multi-layer, microporous polyolefin membraneis a three-layer membrane, the thickness ratio of the layers expressedas (first microporous layer/second microporous layer/third microporouslayer) can range, e.g., from about 1/(0.015 to 0.95)/1, or from about1/(0.02 to 0.8)/1, with the thicknesses of the first and thirdmicroporous layers normalized to 1. When this thickness ratio is lessthan about 1/0.015/1, it is more difficult to produce a multi-layer,microporous polyolefin membrane having a relatively high meltdowntemperature. When this thickness ratio is greater than about 1/0.95/1,it can be more difficult to produce a battery separator having suitableelectrolytic solution absorption characteristics. The total thicknessmulti-layer, microporous polyolefin membrane is not critical. Forexample, the multi-layer, microporous polyolefin membrane can have atotal thickness ranging from about 3 μm to about 200 μm, or from about 5μm to 50 μm, or from about 10 μm to about 35 μm.

(E) Properties of the Multi-layer, Microporous Polyolefin Membrane

In an embodiment, the multi-layer, microporous polyolefin membrane hasone or more of the following properties. In another embodiment, themulti-layer, microporous polyolefin membrane has all of the followingproperties.

(1) Air Permeability of about 20 Seconds/100 cm³ to about 400Seconds/100 cm³ (Converted to Value at 20-μm Thickness)

When the air permeability of the multi-layer, microporous polyolefinmembrane (as measured according to JIS P8117) ranges from about 20seconds/100 cm³ to about 400 seconds/100 cm³, it is less difficult toform batteries having the desired charge storage capacity and desiredcyclability. When the air permeability is less than about 20 seconds/100cm³, it is more difficult to produce a battery having the desiredshutdown characteristics, particularly when the temperatures inside thebatteries are elevated. Air permeability P₁ measured on a multi-layer,microporous membrane having a thickness T₁ according to JIS P8117 can beconverted to air permeability P₂ at a thickness of 20 μm by the equationof P₂=(Pi×20)/TI.

(2) Porosity of 25 to 80%

When the porosity is less than 25%, the multi-layer, microporouspolyolefin membrane generally does not exhibit the desired airpermeability for use as a battery separator. When the porosity exceeds80%, it is more difficult to produce a battery separator of the desiredstrength, which can increase the likelihood of internal electrodeshort-circuiting.

(3) Pin Puncture Strength of 2,000 mN or More (Converted to the Value at20-μm Thickness)

The pin puncture strength (converted to the value at 20-μm thickness) isthe maximum load measured when the multi-layer, microporous polyolefinmembrane is pricked with a needle 1 mm in diameter with a spherical endsurface (radius R of curvature: 0.5 mm) at a speed of 2 mm/second. Whenthe pin puncture strength of the multi-layer, microporous polyolefinmembrane is less than 2,000 mN/20 μm, it is more difficult to produce abattery having the desired mechanical integrity, durability, andtoughness.

(4) Tensile Rupture Strength of 49,000 kPa or More

When the tensile strength according to ASTM D882 of the multi-layer,microporous polyolefin membrane is at least about 49,000 kPa in bothmachine and transverse directions, it is less difficult to produce abattery of the desired mechanical strength. The tensile strength of themulti-layer, microporous polyolefin membrane is preferably 80,000 kPa ormore.

(5) Tensile Rupture Elongation of 100% or More

When the tensile elongation according to ASTM D882 of the multi-layer,microporous polyolefin membrane is 100% or more in both machine andtransverse directions, it is less difficult to produce a battery havingthe desired mechanical integrity, durability, and toughness.

(6) Heat Shrinkage Ratio of 12% or Less

When the heat shrinkage ratio measured after holding the multi-layer,microporous polyolefin membrane at a membrane temperature of about 105°C. for 8 hours exceeds 12% in both machine and transverse directions, itis more difficult to produce a battery that will not exhibit internalshort-circuiting when the heat generated in the battery results in theshrinkage of the separators.

(7) Meltdown Temperature of 150° C. or Higher

In an embodiment, the meltdown temperature can range from about 150° C.to about 190° C. One way to measure meltdown temperature involvesdetermining the temperature at which a multi-layer, microporouspolyolefin membrane test piece of 3 mm in the machine direction and 10mm in the transverse direction is broken by melting, under theconditions that the test piece is heated from room temperature at aheating rate of 5° C./minute while drawing the test piece in the machinedirection under a load of 2 g.

(8) Surface Roughness of 3×10² nm or More

The surface roughness of the first microporous layer material measuredby an atomic force microscope (AFM) in a dynamic force mode is about3×102 nm or more (based on maximum height differences on the surface ofthe layer). In another embodiment, the surface roughness is preferably3.5×10² nm or more.

[5] Battery Separator

In an embodiment, the battery separator comprises the multi-layermicroporous polyolefin membrane. The thickness of the battery separatoris not critical, and can range from, e.g., about 3 μm to about 200 μm,or from about 5 μm to about 50 μm. In an embodiment, the batteryseparator has a thickness ranging from about 10 μm to about 35 μm. Thoseskilled in the art are aware that the thickness of the separator dependson the type and intended use of the. battery.

[6] Battery

In an embodiment, the multi-layer, microporous polyolefin membrane canbe used as a separator for primary and secondary batteries such aslithium ion batteries, lithium-polymer secondary batteries,nickel-hydrogen secondary batteries, nickel-cadmium secondary batteries,nickel-zinc secondary batteries, silver-zinc secondary batteries, andparticularly for lithium ion secondary batteries. Explanations will bemade below on the lithium ion secondary batteries.

The lithium secondary battery comprises a cathode, an anode, and aseparator located between the anode and the cathode. The separatorgenerally contains an electrolytic solution (electrolyte). The electrodestructure is not critical, and conventional electrode structures can beused. The electrode structure may be, for instance, a coin type in whicha disc-shaped cathode and anode are opposing, a laminate type in which aplanar cathode and anode are alternately laminated with at least oneseparator situated between the anode and the cathode, a toroidal type inwhich ribbon-shaped cathode and anode are wound, etc.

The cathode generally comprises a current collector, and acathodic-active material layer capable of absorbing and discharginglithium ions, which is formed on the current collector. Thecathodic-active materials can be, e.g., inorganic compounds such astransition metal oxides, composite oxides of lithium and transitionmetals (lithium composite oxides), transition metal sulfides, etc. Thetransition metals can be, e.g., V, Mn, Fe, Co, Ni, etc. In anembodiment, the lithium composite oxides are lithium nickelate, lithiumcobaltate, lithium manganate, laminar lithium composite oxides based onα-NaFeO₂, etc. The anode generally comprises a current collector, and anegative-electrode active material layer formed on the currentcollector. The negative-electrode active materials can be, e.g.,carbonaceous materials such as natural graphite, artificial graphite,cokes, carbon black, etc.

The electrolytic solutions can be obtained by dissolving lithium saltsin organic solvents. The choice of solvent and/or lithium salt is notcritical, and conventional solvents and salts can be used. The lithiumsalts can be, e.g., LiClO₄, LiPF₆, LiAsF₆, LiSbF₆, LiBF₄, LiCF₃SO₃,LiN(CF₃SO₂)₂, LiC(CF₃SO₂)₃, Li₂B₁₀Cl₁₀, LiN(C₂F₅SO₂)₂, LiPF₄(CF₃)₂,LiPF₃(C₂F₅)₃, lower aliphatic carboxylates of lithium, LiAlCl₄, etc. Thelithium salts may be used alone or in combination. The organic solventscan be organic solvents having relatively high boiling points (comparedto the battery's shut-down temperature) and high dielectric constants.Suitable organic solvents include ethylene carbonate, propylenecarbonate, ethylmethyl carbonate, γ-butyrolactone, etc.; organicsolvents having low boiling points and low viscosity such astetrahydrofuran, 2-methyltetrahydrofuran, dimethoxyethane, dioxolane,dimethyl carbonate, diethyl carbonate, and the like, including mixturesthereof. Because the organic solvents generally having high dielectricconstants generally also have a high viscosity, and vice versa, mixturesof high- and low-viscosity solvents can be used.

When the battery is assembled, the separator is generally impregnatedwith the electrolytic solution, so that the separator (multi-layer,microporous membrane) is provided with ion permeability. The choice ofimpregnation method is not critical, and conventional impregnationmethods can be used. For example, the impregnation treatment can beconducted by immersing the multi-layer, microporous membrane in anelectrolytic solution at room temperature.

The method selected for assembling the battery is not critical, andconventional battery-assembly methods can be used. For example, when acylindrical battery is assembled, a cathode sheet, a separator formed bythe multi-layer, microporous membrane and an anode sheet are laminatedin this order, and the resultant laminate is wound to a toroidal-typeelectrode assembly. A second separator might be needed to preventshort-circuiting of the toroidal windings. The resultant electrodeassembly can be deposited into a battery can and then impregnated withthe above electrolytic solution, and a battery lid acting as a cathodeterminal provided with a safety valve can be caulked to the battery canvia a gasket to produce a battery.

[7] EXAMPLES

The present invention will be explained in more detail referring to thefollowing examples.

Example 1 (1) Preparation of First Polyolefin Solution

Dry-blended were 99.8 parts by mass of a first polyethylene compositioncomprising 2% by mass of ultra-high-molecular-weight polyethylene(UHMWPE) having a weight-average molecular weight (Mw) of 2.0×10⁶, amolecular weight distribution (Mw/Mn) of 8, a melting point (Tm) of 135°C., and a crystal dispersion temperature (Tcd) of 100° C., and 98% bymass of high-density polyethylene (HDPE) having Mw of 3.0×10⁵ and Mw/Mnof 8.6, Tm of 135° C., and Tcd of 100° C., and 0.2 parts by mass oftetrakis[methylene-3-(3,5-ditertiary-butyl-4-hydroxyphenyl)-propionate]methaneas an antioxidant. The first polyethylene composition had Mw of 3.3×10⁵and Mw/Mn of 9.4, Tm of 135° C., and Tcd of 100° C. 40 parts by mass ofthe resultant mixture was charged into a strong-blending double-screwextruder having an inner diameter of 58 mm and L/D of 52.5, and 60 partsby mass of liquid paraffin [50 cst (40° C.)] was supplied to thedouble-screw extruder via a side feeder. Melt-blending was conducted at210° C. and 200 rpm to prepare a first polyolefin solution.

(2) Preparation of Second Polyolefin Solution

Dry-blended were 99.8 parts by mass of UHMWPE, and 0.2 parts by mass ofthe above antioxidant. 5 parts by mass of the mixture was charged intothe same strong-blending double-screw extruder as above, and 95 parts bymass of the same liquid paraffin as above was supplied to thedouble-screw extruder via a side feeder. Melt-blending was conducted at210° C. and 200 rpm to prepare a second polyolefin solution.

The Mw and Mw/Mn of each UHMWPE and HDPE were measured by a gelpermeation chromatography (GPC) method under the following conditions.

Measurement apparatus: GPC-150C available from Waters Corporation,

Column: Shodex UT806M available from Showa Denko K.K., Columntemperature: 135° C.,

Solvent (mobile phase): o-dichlorobenzene,

Solvent flow rate: 1.0 ml/minute,

-   -   Sample concentration: 0.1% by weight (dissolved at 135° C. for 1        hour), Injected amount: 500

Detector: Differential Refractometer available from Waters Corp., andCalibration curve: Produced from a calibration curve of asingle-dispersion, standard polystyrene sample using a predeterminedconversion constant.

(3) Production of the Microporous Polyolefin Membrane

The first and second polyolefin solutions were supplied from theirdouble-screw extruders to a three-layer-sheet-forming T-die at 210° C.,forming a laminate of first solution layer/second solution layer/firstsolution layer at a thickness ratio of 1/0.2/1 was extruded. Theextrudate was cooled while passing through cooling rolls controlled at0° C., to form a three-layer, gel-like sheet. Using a tenter-stretchingmachine, the three-layer, gel-like sheet was simultaneously biaxiallystretched at 116.5° C. to 5 fold in both machine and transversedirections. The stretched three-layer, gel-like sheet was fixed to analuminum frame of 20 cm×20 cm, and immersed in a bath of methylenechloride controlled at a temperature of 25° C. to remove the liquidparaffin with vibration of 100 rpm for 3 minutes. The resulting membranewas air-cooled at room temperature. The dried membrane was re-stretchedby a batch-stretching machine to a magnification of 1.4 fold in atransverse direction at 127° C. The re-stretched membrane, whichremained fixed to the batch-stretching machine, was heat-set at 127° C.for 10 minutes to produce a three-layer, microporous polyolefinmembrane.

Example 2 (1) Preparation of First Polyolefin Solution

A first polyolefin solution was prepared in the same manner as inExample 1.

(2) Preparation of Second Polyolefin Solution

Dry-blended were 100 parts by mass of a polyethylene compositioncomprising 50% by mass of UHMWPE, and 50% by mass of HDPE, and 0.2 partsby mass of the above antioxidant. 15 parts by mass of the mixture wascharged into the double-screw extruder, and 85 parts by mass of theliquid paraffin was supplied to the double-screw extruder via a sidefeeder. Melt-blending was conducted at 210° C. and 200 rpm to prepare asecond polyolefin solution. The second polyethylene composition had Mwof 1.2×10⁶ and Mw/Mn of 18.7, Tm of 135° C., and Tcd of 100° C.

(3) Production of the Microporous Polyolefin Membrane

A three-layer, microporous polyolefin membrane was produced in the samemanner as in Example 1, except that the first and second polyolefinsolutions were extruded at a thickness ratio of first solutionlayer/second solution layer/first solution layer=1/1/1, that thestretching temperature of the gel-like sheet was 117° C., and that thestretching and heat-setting temperatures of the multi-layer, microporouspolyolefin membrane were 126° C.

Example 3 (1) Preparation of First Polyolefin Solution

A first polyolefin solution was prepared in the same manner as inExample 1.

(2) Preparation of Second Polyolefin Solution

Dry-blended were 100 parts by mass of a second polyolefin comprising 30%by mass of UHMWPE having Mw of 5.0×10⁶, Mw/Mn of 8.2, Tm of 135° C., andTcd of 100° C., and 70% by mass of HDPE, and 0.2 parts by mass of theabove antioxidant. 25 parts by mass of the mixture was charged into adouble-screw extruder, and 75 parts by mass of liquid paraffin wassupplied to the double-screw extruder via a side feeder. Melt-blendingwas conducted at 210° C. and 200 rpm to prepare a second polyolefinsolution. The second polyethylene composition had Mw of 8.1×10⁵, Mw/Mnof 17.2, Tm of 135° C., and Tcd of 100° C.

(3) Production of the Microporous Polyolefin Membrane

A three-layer, microporous polyolefin membrane was produced in the samemanner as in Example 1, except that the first and second polyolefinsolutions were extruded at a thickness ratio of first solutionlayer/second solution layer/first solution layer=1/0.5/1, and that thestretching temperature of the gel-like sheet was 117° C.

Example 4

A three-layer, microporous polyolefin membrane was produced in the samemanner as in Example 1, except that the stretching temperature of thegel-like sheet was 117.5° C., and that the multi-layer, microporouspolyolefin membrane was re-stretched to a magnification of 1.6 fold.

Example 5 (1) Preparation of First Polyolefin Solution

A first polyolefin solution was prepared in the same manner as inExample 1 except for using a composition comprising 2% by mass ofUHMWPE, 93% by mass of HDPE and 5% by mass of PP having Mw of 5.3×10⁵.The first polyethylene composition had Mw of 3.3×10⁵ and Mw/Mn of 9.5,Tm of 135° C., and Tcd of 100° C. The Mw of PP was measured by a GPCmethod as above.

(2) Preparation of Second Polyolefin Solution

A second polyolefin solution was prepared in the same manner as in 25Example 1.

(3) Production of the Microporous Polyolefin Membrane

A three-layer, microporous polyolefin membrane was produced in the samemanner as in Example 1, except that the first and second polyolefinsolutions were extruded at a thickness ratio of first solutionlayer/second solution layer/first solution layer 1/0.5/1, and that thestretching temperature of the gel-like sheet was 117.5° C.

Example 6 (1) Preparation of First Polyolefin Solution

A first polyolefin solution was prepared in the same manner as inExample 1.

(2) Preparation of Second Polyolefin Solution

A second polyolefin solution was prepared in the same manner as inExample 2, except for setting its concentration at 20% by mass.

(3) Production of the Microporous Polyolefin Membrane

The first and second polyolefin solutions were supplied from theirdouble-screw extruders to a two-layer-extruding T-die, andsimultaneously extruded therefrom at a thickness ratio of first solutionlayer/second solution layer=1/1, cooled by cooling rolls controlled at0° C. while reeling up, to form a two-layer, gel-like sheet. Thetwo-layer, gel-like sheet was simultaneously biaxially stretched at117.5° C., washed, dried by air, re-stretched, and heat-set in the samemanner as in Example 1 to form a two-layer, microporous polyolefinmembrane.

Example 7 (1) Preparation of First Polyolefin Solution

Dry-blended were 100 parts by mass of HDPE, and 0.2 parts by mass of theabove antioxidant. 40 parts by mass of the resultant mixture was chargedinto the same strong-blending double-screw extruder as above, and 60parts by mass of the same liquid paraffin as above was supplied to thedouble-screw extruder via a side feeder. Melt-blending was conducted at210° C. and 200 rpm to prepare a first polyolefin solution.

(2) Preparation of Second Polyolefin Solution

A second polyolefin solution was prepared in the same manner as inExample 2.

(3) Production of the Microporous Polyolefin Membrane

A three-layer, microporous polyolefin membrane was produced in the samemanner as in Example 1, except that the first and second polyolefinsolutions were extruded at a thickness ratio of first solutionlayer/second solution layer/first solution layer=1/0.5/1, and that thestretching temperature of the gel-like sheet was 117.5° C.

Comparative Example 1

A three-layer, microporous polyolefin membrane was produced in the samemanner as in Example 1, except that the same second polyolefin solutionas in Example 1 except that its concentration was 7% by mass was used,that the first and second polyolefin solutions were extruded at athickness ratio of first solution layer/second solution layer/firstsolution layer=1/2/1, that the stretching temperature of the gel-likesheet was 118° C., and that the stretching and heat-setting temperaturesof the multi-layer, microporous polyolefin membrane were 128° C.

Comparative Example 2

A three-layer, microporous polyolefin membrane was produced in the samemanner as in Example 1, except that the first and second polyolefinsolutions were extruded at a thickness ratio of first solutionlayer/second solution layer/first solution layer=1/0.5/1, that thestretching temperature of the multi-layer, gel-like sheet was 118° C.,and that the stretching and heat-setting temperatures of themulti-layer, microporous polyolefin membrane were 129° C.

Comparative Example 3

A microporous polyethylene membrane was produced in the same manner asin Example 1, except that only the first polyolefin solution was used,that the gel-like sheet was stretched at 118.5° C., and that themicroporous polyolefin membrane was stretched at 129° C. and heat-set at129° C. for 12 seconds.

Comparative Example 4

Dry-blended were 100 parts by mass of 18% by mass of UHMWPE having Mw of2.0×10⁶, and Mw/Mn of 8, and 82% by mass of HDPE having Mw of 3.0×10⁵,and Mw/Mn of 13.5, and 0.2 parts by mass of the above antioxidant. 30parts by mass of the mixture was charged into a double-screw extruder,and 70 parts by mass of liquid paraffin was supplied to the double-screwextruder via a side feeder. Melt-blending was conducted at 210° C. and200 rpm to prepare a second polyolefin solution. The second polyethylenecomposition had Mw of 6.5×10⁵ and Mw/Mn of 20.9. A microporouspolyethylene membrane was produced in the same manner as in Example 1,except that only the second polyolefin solution was used, that thegel-like sheet was stretched at 115° C., and that the polyethylenmembrane was stretched at 124.5° C. and heat-set at 124.5° C. for 12seconds.

The properties of the (triple-layer) microporous polyolefin membranesobtained in Examples 1-7 and Comparative Examples 1-4 were 15 measuredby the following methods. The results are shown in Table 1.

(1) Average Thickness (μm)

The thickness of each multi-layer, microporous membrane was measured bya contact thickness meter at 5 mm machine direction intervals over thewidth of 30 cm, and averaged.

(2) Layer Thickness Ratio

Three membranes obtained by peeling each three-layer, microporouspolyolefin membrane were measured with respect to thickness by a contactthickness meter over a width of 30 cm at a machine interval of 10 mm,and the measured thickness was averaged. The thickness ratio wascalculated from the average thickness of each membrane.

(3) Air Permeability (sec/100 cm³/20 μm)

Air permeability P₁ measured on each (triple-layer) microporouspolyolefin membrane having a thickness T₁ according to JIS P8117 wasconverted to air permeability P₂ at a thickness of 20 μm by the equationof P₂=(P₁×20)/T₁.

(4) Porosity (%)

Measured by conventional weight methods.

(5) Pin Puncture Strength (mN/20 μm)

The maximum load was measured, when each multi-layer, microporousmembrane having a thickness of T₁ was pricked with a needle of 1 mm indiameter with a spherical end surface (radius R of curvature: 0.5 mm) ata rate of 2 mm/second. The measured maximum load L₁ was converted to themaximum load L₂ at a thickness of 20 μm by the equation ofL₂=(L₁×20)/T₁, and used as pin puncture strength.

(6) Tensile Rupture Strength and Tensile Rupture Elongation

Measured on a 10-mm-wide rectangular test piece according to ASTM-D882.

(7) Heat Shrinkage Ratio (%)

The shrinkage ratio of each multi-layer, microporous membrane wasmeasured three times in both machine and transverse directions aftermaintaining a membrane temperature 105° C. for 8 hours, and averagingthe measured shrinkages.

(8) Meltdown Temperature (° C.)

Meltdown temperature was measured using a thermomechanical analyzer(TMA/SS6000 available from Seiko Instruments Inc.). The measurement wasmade as follows: a membrane test piece TP of 10 mm (TD) and 3 mm (MD)was heated from room temperature at a rate of 5° C./minute under a loadof 2 g according to the method shown in FIG. 2. The temperature at whichthe test piece TP elongated by 50% of the original length (100%) at roomtemperature was used as a measure of the “meltdown temperature.”

(9) Pore Size Distribution of First Microporous Layer

The pore size distribution of a first microporous layer membraneconstituting the two or three-layer, microporous polyolefin membrane wasdetermined from a pore size distribution curve obtained by mercuryintrusion porosimetry. Pore size distribution of microporous polyolefinmembrane is measured by a device “Poresizer Type 9320, manufacturing byMicrometritics Ltd.” An area of measurement pressure ranges from about3.7 kPa to about

5 207 MPa. A volume of a cell is 15 cm³. A value of contact angle andsurface tension for mercury were used 141.3° and 484 dyn/cmrespectively.

(10) Pore Volume Ratio in First Microporous Layer

Calculated from S₁/S₂ shown in FIG. 1.

(11) Surface Roughness

The maximum height difference of a surface of a first microporous layermeasured by AFM in a dynamic force mode (DFM) was used as surfaceroughness.

(12) Electrolytic Solution Absorption Rate

Using a kinetic-surface-tension-measuring apparatus (DCAT21 withhigh-precision electronic balance, available from Eko Instruments Co.,ltd.), a triple-layer, microporous polyolefin membrane sample wasimmersed in an electrolytic solution (electrolyte: LiPF₆, electrolyteconcentration: 1 mol/L, solvent: ethylene carbonate/dimethyl carbonateat a volume ratio of 3/7) kept at 18° C., to determine an electrolyticsolution absorption rate by the formula of [increased weight (g) ofmicroporous polyolefin membrane/weight (g) of microporous polyolefinmembrane before absorption]. The absorption rate is expressed by arelative ratio, assuming that the absorption rate (g/g) of themicroporous polyolefin membrane of Comparative Example 1 is 1.

(13) Thickness Variation Ratio After Heat Compression (%)

A microporous polyolefin membrane sample was sandwiched by a pair ofhighly flat plates, and heat-compressed by a press machine under apressure of 2.2 MPa (22 kgf/cm²) at 90° C. for 5 minutes, to determinean average thickness in the same manner as above. A thickness variationratio was calculated by the formula of (average thickness aftercompression−average thickness before compression)/(average thicknessbefore compression)×100.

(14) Air permeability After Heat Compression (sec/100 cm3/

Each microporous polyolefin membrane having a thickness of T₁ washeat-compressed under the above conditions, and measured with respect toair permeability P₁ according to JIS P8117. The measured airpermeability P₁ was converted to air permeability P₂ at a thickness of20 μm by the equation of P₂=(P₁×20)/T₁.

TABLE 1 No. Example 1 Example 2 Example 3 First Polyolefin UHMWPEMw⁽¹⁾/MWD⁽²⁾/% by mass 2.0 × 10⁶/8/2 2.0 × 10⁶/8/2 2.0 × 10⁶/8/2 HDPEMw/MWD/% by mass 3.0 × 10⁵/8.6/98 3.0 × 10⁵/8.6/98 3.0 × 10⁵/8.6/98 PEComposition Mw/MWD 3.3 × 10⁵/9.4 3.3 × 10⁵/9.4 3.3 × 10⁵/9.4 Tm(°C.)⁽³⁾/Tcd (° C.)⁽⁴⁾ 135/100 135/100 135/100 PP Mw/% by mass —/— —/— —/—Second Polyolefin UHMWPE Mw/MWD/% by mass 2.0 × 10⁶/8/100 2.0 × 10⁶/8/505.0 × 10⁶/8.2/30 HDPE Mw/MWD/% by mass —/—/— 3.0 × 10⁵/8.6/50 3.0 ×10⁵/8.6/70 PE Composition Mw/MWD —/— 1.2 × 10⁶/18.7 8.1 × 10⁵/17.2 Tm(°C.)/Tcd (° C.) —/— 135/100 135/100 Production ConditionsConcentration⁽⁵⁾ (% by mass) 40/5  40/15 40/25 Simultaneous ExtrusionLayer Structure⁽⁶⁾ (I)/(II)/(I) (I)(II)(I) (I)(II)(I) Layer ThicknessRatio⁽⁷⁾ 1/0.2/1 1/1/1 1/0.5/1 Stretching of Multi-Layer, Gel-Like SheetTemperature (° C.) 116.5 117 117 Magnification (MD × TD) 5 × 5 5 × 5 5 ×5 Stretching of Multi-Layer, Microporous Membrane Temp. (°C.)/Direction/Magnification (folds) 127/TD/1.4 126/TD/1.4 127/TD/1.4Heat-Setting Temperature (° C.)/Time (minute) 127/10  126/10  127/10 Properties of Multi-Layer, Microporous Membrane Average Thickness (μm)21.5 24.5 20.2 Layer Thickness Ratio⁽⁷⁾ 1/0.02/1 1/0.2/1 1/0.2/1 AirPermeability (sec/100 cm³/20 μm) 192 335 185 Porosity (%) 44.9 46.7 45.1Pin Puncture Strength (mN/20 μm) 5,684 6,624.8 6,076 Tensile RuptureStrength (kPa) in MD/TD 126,332/164,836 139,748/169,540 122,402/159,936Tensile Rupture Elongation (%) in MD/TD 160/180 165/175 145/175 HeatShrinkage Ratio (%) in MD/TD 4.9/5.2 6.3/5.9   6/6.2 MeltdownTemperature (° C.) 158 159 161 Higher-Order Structure Peaks (μm) in PoreSize Distribution⁽⁸⁾ 0.05/0.1/0.7/1 0.05/0.1/0.7/1 0.05/0.1/0.7/1 PoreVolume Ratio⁽⁹⁾ 1.13 1.63 1.30 Surface Rouglmess⁽¹⁰⁾ (nm) 5.8 × 10² 5.8× 10² 5.8 × 10² Electrolytic solution Absorption Rate 3.8 2.9 3.6Thickness Variation Ratio After −18 −19 −20 Heat Compression (%) AirPermeability After Heat Compression 590 722 523 (sec/100 cm³/20 μm) No.Example 4 Example 5 Example 6 First Polyolefin UHMWPE Mw⁽¹⁾/MWD⁽²⁾/% bymass 2.0 × 10⁶/8/2 2.0 × 10⁶/8/2 2.0 × 10⁶/8/2 HDPE Mw/MWD/% by mass 3.0× 10⁵/8.6/98 3.0 x 10⁵/8.6/93 3.0 × 10⁵/8.6/98 PE Composition Mw/MWD 3.3× 10⁵/9.4 3.3 × 10⁵/9.5 3.3 × 10⁵/9.4 Tm(° C.)⁽³⁾/Tcd (° C.)⁽⁴⁾ 135/100135/100 135/100 PP Mw/% by mass —/— 5.3 × 10⁵/5 —/— Second PolyolefinUHMWPE Mw/MWD/% by mass 2.0 × 10⁶/8/100 2.0 x 10⁶/8/100 2.0 × 10⁶/8/50HDPE Mw/MWD/% by mass —/—/— —/—/— 3.0 × 10⁵/8.6/50 PE Composition Mw/MWD—/— —/— 1.2 × 10⁶/18.7 Tm(° C.)/Tcd (° C.) —/— —/— 135/100 ProductionConditions Concentration⁽⁵⁾ (% by mass) 40/5  40/5  40/20 SimultaneousExtrusion Layer Structure⁽⁶⁾ (I)/(II)/(I)⁽⁸⁾ (I)(II)(I) (I)(II)(I) LayerThickness Ratio⁽⁷⁾ 1/0.2/1 1/0.5/1 1/1 Stretching of Multi-Layer,Gel-Like Sheet Temperature (° C.) 117.5 117.5 117.5 Magnification (MD ×TD) 5 × 5 5 × 5 5 × 5 Stretching of Multi-Layer, Microporous MembraneTemp. (° C.)/Direction/Magnification (folds) 127/TD/1.6 127/TD/1.4127/TD/1.4 Heat-Setting Temperature (° C.)/Time (minute) 127/10 127/10127/10 Properties of Multi-Layer, Microporous Membrane Average Thickness(μm) 21 20.5 19.2 Layer Thickness Ratio⁽⁷⁾ 1/0.02/1 1/0.03/1 0.5/1⁽¹¹⁾Air Permeability (sec/100 cm³/20 μm) 159 195 197 Porosity (%) 45.8 45.545.4 Pin Puncture Strength (mN/20 μm) 5,448.8 5,635 4,802 TensileRupture Strength (kPa) in MD/TD 123,970/173,950 125,440/158,760122,500/136,220 Tensile Rupture Elongation (%) in MD/TD 165/155 155/175160/175 Heat Shrinkage Ratio (%) in MD/TD 4.5/6.0 4.8/4.9 3.8/5.5Meltdown Temperature (° C.) 158 158 158 Higher-Order Structure Peaks(μm) in Pore Size Distribution⁽⁸⁾ 0.05/0.1/0.8/1.1 0.05/0.1/0.7/10.05/0.1/0.7/1 Pore Volume Ratio⁽⁹⁾ 0.91 1.18 1.74 Surface Rouglmess⁽¹⁰⁾(nm) 6.0 × 10² 5.2 × 10² 5.6 × 10² Electrolytic solution Absorption Rate3.9 3.5 3 Thickness Variation Ratio After −22 −18 −19 Heat Compression(%) Air Permeability After Heat Compression 492 575 624 (sec/100 cm³/20μm) No. Example 7 Com. Ex. 1 Com. Ex. 2 First Polyolefin UHMWPEMw⁽¹⁾/MWD⁽²⁾/% by mass —/—/— 2.0 × 10⁶/8/2 2.0 × 10⁶/8/2 HDPE Mw/MWD/%by mass 3.0 × 10⁵/8.6/100 3.0 × 10⁵/8.6/98 3.0 × 10⁵/8.6/98 PEComposition Mw/MWD —/— 3.3 × 10⁵/9.4 3.3 × 10⁵/9.4 Tm(° C.)⁽³⁾/Tcd (°C.)⁽⁴⁾ —/— 135/100 135/100 PP Mw/% by mass —/— —/— —/— Second PolyolefinUHMWPE Mw/MWD/% by mass 2.0 × 10⁶/8/50 2.0 × 10⁶/8/100 2.0 × 10⁶/8/100HDPE Mw/MWD/% by mass 3.0 × 10⁵/8.6/50 —/—/— —/—/— PE Composition Mw/MWD1.2 × 10⁶/18.7 —/— —/— Tm(° C.)/Tcd (° C.) 135/100 —/— —/— ProductionConditions Concentration⁽⁵⁾ (% by mass) 40/15 40/7  40/5  SimultaneousExtrusion Layer Structure⁽⁶⁾ (I)/(II)/(I) (I)/(II)/(I) (I)/(II)/(I)Layer Thickness Ratio⁽⁷⁾ 1/0.5/1 1/2/1 1/0.05/1 Stretching ofMulti-Layer, Gel-Like Sheet Temperature (° C.) 117.5 118 118Magnification (MD × TD) 5 × 5 5 × 5 5 × 5 Stretching of Multi-Layer,Microporous Membrane Temp. (° C.)/Direction/Magnification (folds)127/TD/1.4 128/TD/1.4 129/TD/1.4 Heat-Setting Temperature (° C.)/Time(minute) 127/10 128/10 129/10 Properties of Multi-Layer, MicroporousMembrane Average Thickness (μm) 19.2 23.7 20.1 Layer Thickness Ratio⁽⁷⁾1/0.1/1 1/1/1 1/0.01/1 Air Permeability (sec/100 cm³/20 μm) 170 227 234Porosity (%) 44.5 42.5 41.5 Pin Puncture Strength (mN/20 μm) 5,7824,978.4 4,645.2 Tensile Rupture Strength (kPa) in MD/TD 118,580/148,960106,232/122,990 117,600/154,840 Tensile Rupture Elongation (%) in MD/TD130/160 175/190 170/180 Heat Shrinkage Ratio (%) in MD/TD 6.5/5.53.3/3.8 3.5/4   Meltdown Temperature (° C.) 157 159 148 Higher-OrderStructure Peaks (μm) in Pore Size Distribution⁽⁸⁾ 0.05/0.1/0.7/10.05/0.1/0.7/1 0.05/0.1/0.7/1 Pore Volume Ratio⁽⁹⁾ 1.66 2.70 1.10Surface Rouglmess⁽¹⁰⁾ (nm) 5.0 × 10² 5 × 10² 5.8 × 10² Electrolyticsolution Absorption Rate 3.2 1.8 3.6 Thickness Variation Ratio After −19−23 −19 Heat Compression (%) Air Permeability After Heat Compression 654824 554 (sec/100 cm³/20 μm) No. Com. Ex. 3 Com. Ex. 4 First PolyolefinUHMWPE Mw⁽¹⁾/MWD⁽²⁾/% by mass 2.0 × 10⁶/8/2 —/—/— HDPE Mw/MWD/% by mass3.0 × 10⁵/8.6/98 —/—/ — PE Composition Mw/MWD 3.3 × 10⁵/9.4 —/— Tm(°C.)⁽³⁾/Tcd (° C.)⁽⁴⁾ 135/100 —/— PP Mw/% by mass —/— —/— SecondPolyolefin UHMWPE Mw/MWD/% by mass —/—/— 2.0 × 10⁶/8/18 HDPE Mw/MWD/% bymass —/—/— 3.0 × 10⁵/13.5/82 PE Composition Mw/MWD —/— 6.5 × 10⁵/20.9Tm(° C.)/Tcd (° C.) —/— 135/100 Production Conditions Concentration⁽⁵⁾(% by mass) 40/— —/30 Simultaneous Extrusion Layer Structure⁽⁶⁾ — —Layer Thickness Ratio⁽⁷⁾ — — Stretching of Multi-Layer, Gel-Like SheetTemperature (° C.) 118.5 115 Magnification (MD × TD) 5 × 5 5 × 5Stretching of Multi-Layer, Microporous Membrane Temp. (°C.)/Direction/Magnification (folds) 129/TD/1.4 124.5/TD/1.4 Heat-SettingTemperature (° C.)/Time (minute) 129/0.2  124.5/0.2  Properties ofMulti-Layer, Microporous Membrane Average Thickness (μm) 19.5 20 LayerThickness Ratio⁽⁷⁾ — — Air Permeability (sec/100 cm³/20 μm) 212 420Porosity (%) 41.2 37.2 Pin Puncture Strength (mN/20 μm) 4,488.4 4,655Tensile Rupture Strength (kPa) in MD/TD 117,110/155,820 166,600/133,280Tensile Rupture Elongation (%) in MD/TD 175/185 150/230 Heat ShrinkageRatio (%) in MD/TD 3.5/4   7.5/5.5 Meltdown Temperature (° C.) 146 150Higher-Order Structure Peaks (μm) in Pore Size Distribution⁽⁸⁾0.05/0.1/0.7/1 0.045/—/—/— Pore Volume Ratio⁽⁹⁾ 1.10 — SurfaceRouglmess⁽¹⁰⁾ (nm) 5.8 × 10² 2.0 × 10² Electrolytic solution AbsorptionRate 3.8 1.0 Thickness Variation Ratio After −18 −18 Heat Compression(%) Air Permeability After Heat Compression 524 1,525 (sec/100 cm³/20μm) Note: ⁽¹⁾Mw represents weight-average molecular weight. ⁽²⁾Themolecular weight distribution represented by weight-average molecularweight/number-average molecular weight (Mw/Mn). ⁽³⁾Tm represents themelting point of the polyethylene composition. (4)Tcd represents thecrystal dispersion temperature of the polyethylene composition. ⁽⁵⁾Theconcentration of the first polyolefin solution and the concentration ofthe second polyolefin solution. ⁽⁶⁾The layer structure of surfacelayer/inner layer/surface layer, and (I) represents the first polyolefinsolution, and (II) represents the second polyolefin solution. ⁽⁷⁾Thethickness ratio of surface layer/inner layer/surface layer. ⁽⁸⁾First tofourth peaks (pm) in the pore size distribution of the first microporouslayer. ⁽⁹⁾The pore volume ratio in the first microporous layer.⁽¹⁰⁾Surface roughness (maximum height difference) measured by AFM in adynamic force mode (DFM). ⁽¹¹⁾The thickness ratio of the firstmicroporous layer/ the second microporous layer.

As is clear from Table 1, each three-layer, microporous polyolefinmembrane of Examples 1-7 had a structure in which the first microporouslayer had a hybrid structure, thereby exhibiting favorable andwell-balanced electrolytic solution absorption and compressionresistance properties. They further had favorable and well-balancedpermeability, pin puncture strength, tensile rupture strength, tensilerupture elongation, heat shrinkage resistance and meltdown properties.

It is believed that the three-layer, the microporous polyolefin membraneof Comparative Example 1 was poorer than those of Examples 1-7 inelectrolytic solution absorption, because the thickness ratio of thesecond microporous layer to the first microporous layer (firstmicroporous layer/second microporous layer/first microporous layer) wasmore than 1/0.95/1. It is believed that the three-layer, microporouspolyolefin membrane of Comparative Example 2 was poorer than those ofExamples 1-7 in pin puncture strength and meltdown properties, becausethe thickness ratio of the second microporous layer to the firstmicroporous layer (first microporous layer/second microporouslayer/first microporous layer) was less than 1/0.015/1. It is believedthat the non-multi-layer, microporous polyolefin membrane of ComparativeExample 3 was poorer than those of Examples 1-7 in pin puncture strengthand meltdown properties, because the membrane of Comparative Example 3did not have the second microporous layer in which the percentage of theultra-high-molecular-weight polyethylene was 8% or more by mass. It isbelieved that the non-multi-layer, microporous polyolefin membrane ofComparative Example 4 was poorer than those of Examples 1-7 in airpermeability after heat compression and electrolytic solutionabsorption, because the membrane of Comparative Example 4 did not havethe first microporous layer having a hybrid structure.

EFFECT OF THE INVENTION

The invention relates in part to multi-layer, microporous polyolefinmembranes having suitably well-balanced permeability, mechanicalstrength, heat shrinkage resistance, meltdown properties, electrolyticsolution absorption, and compression resistance properties; and tomethods for making and using such membranes. Separators formed from suchmulti-layer, microporous polyolefin membrane provide batteries withsuitable safety, heat resistance, storage properties, and productivity.

1. A method for forming a polyolefin, comprising, (1) combining a firstpolyolefin composition and a first membrane-forming solvent to prepare afirst polyolefin solution, wherein the first polyolefin is produced bycombining resins of (a) a first polyethylene having an Mw that is lessthan 1×10⁶; (b) the first polyethylene and a second polyethylene havinga Mw of at least 1×10⁶, wherein the second polyethylene is present in anamount that does not exceed 7% by mass based on the combined mass of thefirst and second polyethylene; (c) the first polyethylene and a firstpolypropylene, wherein the amount of the polypropylene ranges does notexceed 25% by mass based on the combined mass of the first polyethyleneand the first polypropylene; or (d) the first polyethylene, the secondpolyethylene, and the first polypropylene, wherein the firstpolypropylene is present in an amount that does not exceed 25% by massbased on the combined mass of the first polyethylene, the secondpolyethylene, and the first polypropylene, and wherein the secondpolyethylene is present in an amount that does not exceed 7% by massbased on the combined mass of the first and second polyethylene; and (2)combining a second polyolefin composition and a second membrane-formingsolvent to prepare a second polyolefin solution, wherein the secondpolyolefin composition is produced by combining resins of (a) a fourthpolyethylene having an Mw of at least 1×10⁶; (b) a third polyethylenehaving an Mw that is less than 1×10⁶ and the fourth polyethylene,wherein the fourth polyethylene is present in an amount of at least 8%by mass based on the combined mass of the third and fourth polyethylene;(c) the fourth polyethylene and a second polypropylene wherein thesecond polypropylene is present in an amount that does not exceed 25% bymass based on the combined mass of the fourth polyethylene and thesecond polypropylene; or (d) the third polyethylene, the fourthpolyethylene, and the second polypropylene, wherein second polypropyleneis present in an amount that does not exceed 25% by mass based on thecombined mass of the third polyethylene, the fourth polyethylene, andthe second polypropylene, and the fourth polyethylene is present in anamount of at least 8% by mass based on the combined mass of the thirdand fourth polyethylene.
 2. The method of claim 1, further comprising;(3) extruding at least a portion of the first polyolefin solutionthrough a first die and co-extruding at least a portion of the secondpolyolefin solution through a second die in order to form a multi-layerextrudate, (4) cooling the multi-layer extrudate to form a multi-layer,gel-like sheet, (5) removing the first and second membrane-formingsolvents from the multi-layer, gel-like sheet to form a solvent-removedgel-like sheet, and (6) drying the solvent-removed gel-like sheet inorder to form the multi-layer, microporous polyolefin membrane.
 3. Themethod of claim 1, further comprising; (3) extruding at least a portionof the first polyolefin solution through a first die to make a firstextrudate and extruding at least a portion of the second polyolefinsolution through a second die to make a second extrudate, and thenlaminating the first and second extrudates to make a multi-layerextrudate, (4) cooling the multi-layer extrudate to form a multi-layer,gel-like sheet, (5) removing the first and second membrane-formingsolvents from the multi-layer, gel-like sheet to form a solvent-removedgel-like sheet, and (6) drying the solvent-removed gel-like sheet inorder to form the multi-layer, microporous polyolefin membrane.
 4. Themethod of claim 1, further comprising (3) extruding at least a portionof the first polyolefin solution through a first die to make a firstextrudate, (4) extruding at least a portion of the second polyolefinsolution through a second die to make a second extrudate, (5) coolingfirst and second extrudates to form at least one first gel-like sheetand at least one second gel-like sheet, (6) laminating the first andsecond gel-like sheets to form a multi-layer, gel-like sheet, (7)removing the first and second membrane-forming solvents from themulti-layer, gel-like sheet to form a solvent-removed gel-like sheet,and (8) drying the solvent-removed gel-like sheet in order to form themulti-layer, microporous polyolefin membrane.
 5. The method of claim 1,further comprising: (3) extruding at least a portion of the firstpolyolefin solution through a first die to make a first extrudate, (4)extruding at least a portion of the second polyolefin solution throughat least a second die to make a second extrudate, (5) cooling first andsecond extrudates to form at least one first gel-like sheet and at leastone second gel-like sheet, (6) removing the first and secondmembrane-forming solvents from the first and second gel-like sheets, (7)drying the solvent-removed first and second gel-like sheets to form atleast one first microporous polyolefin membrane and at least one secondmicroporous polyolefin membrane, and (8) laminating the first and secondmicroporous polyolefin membranes in order to form the multi-layer,microporous polyolefin membrane.